[gdb/testsuite] Fix check-read1 FAIL in gdb.tui/corefile-run.exp

With test-case gdb.tui/corefile-run.exp and make target check-read1, I run
into:
...
FAIL: gdb.tui/corefile-run.exp: run until the end
...

In more detail, using -v:
...
PASS: gdb.tui/corefile-run.exp: load corefile
^M+++ _ctl_0x0d
^[[17d+++ _csi_d <<<17>>>
^[[M+++ _csi_M <<<>>>
^[[24d+++ _csi_d <<<24>>>
(INSERT <<(>>
gINSERT <<g>>
dINSERT <<d>>
bINSERT <<b>>
)INSERT <<)>>
 INSERT << >>
FAIL: gdb.tui/corefile-run.exp: run until the end
...

With some debugging code added in wait_for, what happens becomes more clear:
...
	    if {[regexp -- $wait_for $prev]} {
+		verbose -log "\nwait_for: MATCHED line ($_cur_y): \"$prev\""
+		verbose -log "wait_for: AGAINST regexp: \"$wait_for\""
...

In corefile-run.exp, we execute:
...
Term::command "run"
...
and in proc Term::command, we send the command, and then call wait_for:
...
    proc command {cmd} {
        send_gdb "$cmd\n"
        wait_for [string_to_regexp $cmd]
    }
...
which first waits for the command string, and then for the prompt.

In this case however, the matching of the command string triggers on a
previous line:
...
wait_for: MATCHED line (16): \
  "(gdb) core-file corefile-run.core[New LWP 6426] <lots-of-spaces>"
wait_for: AGAINST regexp: "run"
...
and from there on things go out of sync, eventually resulting in the FAIL.

Fix this in proc command by more precisely specifying the expected pattern:
adding a ^$gdb_prompt prefix.

Add a command_no_prompt_prefix variant to use for initial terminal commands
where there's no prompt yet.

Tested gdb.tui/*.exp on x86_64-linux, with make target check and check-read1.

gdb/testsuite/ChangeLog:

2020-03-13  Tom de Vries  <tdevries@suse.de>

	* lib/tuiterm.exp (Term::command_no_prompt_prefix): New proc.
	(Term::command): Use prompt prefix.
	(Term::enter_tui): Use command_no_prompt_prefix instead of prefix.
	* gdb.tui/tui-layout-asm-short-prog.exp: Use
	command_no_prompt_prefix instead of prefix.
	* gdb.tui/tui-layout-asm.exp: Same.

4 files changed, 24 insertions, 4 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index e13a230..de419eb 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,12 @@
+2020-03-13  Tom de Vries  <tdevries@suse.de>
+
+	* lib/tuiterm.exp (Term::command_no_prompt_prefix): New proc.
+	(Term::command): Use prompt prefix.
+	(Term::enter_tui): Use command_no_prompt_prefix instead of prefix.
+	* gdb.tui/tui-layout-asm-short-prog.exp: Use
+	command_no_prompt_prefix instead of prefix.
+	* gdb.tui/tui-layout-asm.exp: Same.
+
 2020-03-12  Simon Marchi  <simon.marchi@efficios.com>
 
 	* gdb.base/break-interp.exp: Use foreach_with_prefix.
diff --git a/gdb/testsuite/gdb.tui/tui-layout-asm-short-prog.exp b/gdb/testsuite/gdb.tui/tui-layout-asm-short-prog.exp
index d0b871f..4aa1ba3 100644
--- a/gdb/testsuite/gdb.tui/tui-layout-asm-short-prog.exp
+++ b/gdb/testsuite/gdb.tui/tui-layout-asm-short-prog.exp
@@ -32,7 +32,7 @@ if {![Term::prepare_for_tui]} {
 }
 
 # This puts us into TUI mode, and should display the ASM window.
-Term::command "layout asm"
+Term::command_no_prompt_prefix "layout asm"
 Term::check_box_contents "check asm box contents" 0 0 80 15 "<_start>"
 
 # Record the first line of output, we'll need this later.
diff --git a/gdb/testsuite/gdb.tui/tui-layout-asm.exp b/gdb/testsuite/gdb.tui/tui-layout-asm.exp
index 40f46ea..257321f 100644
--- a/gdb/testsuite/gdb.tui/tui-layout-asm.exp
+++ b/gdb/testsuite/gdb.tui/tui-layout-asm.exp
@@ -30,7 +30,7 @@ if {![Term::prepare_for_tui]} {
 }
 
 # This puts us into TUI mode, and should display the ASM window.
-Term::command "layout asm"
+Term::command_no_prompt_prefix "layout asm"
 Term::check_box_contents "check asm box contents" 0 0 80 15 "<main>"
 
 # Scroll the ASM window down using the down arrow key.  In an ideal
diff --git a/gdb/testsuite/lib/tuiterm.exp b/gdb/testsuite/lib/tuiterm.exp
index 7505821..8c9f97a 100644
--- a/gdb/testsuite/lib/tuiterm.exp
+++ b/gdb/testsuite/lib/tuiterm.exp
@@ -489,7 +489,7 @@ namespace eval Term {
 	    return 0
 	}
 
-	command "tui enable"
+	command_no_prompt_prefix "tui enable"
 	return 1
     }
 
@@ -497,8 +497,19 @@ namespace eval Term {
     # seen in the TUI.  CMD should not end with a newline -- that will
     # be supplied by this function.
     proc command {cmd} {
+	global gdb_prompt
+	send_gdb "$cmd\n"
+	set str [string_to_regexp $cmd]
+	set str "^$gdb_prompt $str"
+	wait_for $str
+    }
+
+    # As proc command, but don't wait for a initial prompt.  This is used for
+    # inital terminal commands, where there's no prompt yet.
+    proc command_no_prompt_prefix {cmd} {
 	send_gdb "$cmd\n"
-	wait_for [string_to_regexp $cmd]
+	set str [string_to_regexp $cmd]
+	wait_for "^$str"
     }
 
     # Return the text of screen line N, without attributes.  Lines are