[gdb/testsuite] Handle shell prompt in batch-preserve-term-settings.exp

On SLE-11, I run into:
...
FAIL: gdb.base/batch-preserve-term-settings.exp: batch run: spawn shell \
  (timeout)
...

The problem is that the shell prompt has PS1="\h:\w> ", but the test expects
a shell prompt ending in a space preceded by either '$' or '#':
...
set shell_prompt_re "\[$#\] "
...

We could easily fix this by adding '>' to shell_prompt_re, but this wouldn't
work for other PS1 setting.

Fix this instead by setting the shell prompt to "gdb-subshell$ " (as in
gdb.base/multi-line-starts-subshell.exp).

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-12-14  Tom de Vries  <tdevries@suse.de>

	PR testsuite/26951
	* gdb.base/batch-preserve-term-settings.exp: Use "gdb-subshell$ " as
	shell prompt.

2 files changed, 12 insertions, 3 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index d5442f9..4bd4521 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,9 @@
+2020-12-14  Tom de Vries  <tdevries@suse.de>
+
+	PR testsuite/26951
+	* gdb.base/batch-preserve-term-settings.exp: Use "gdb-subshell$ " as
+	shell prompt.
+
 2020-12-14  Tom Tromey  <tromey@adacore.com>
 
 	* gdb.ada/nested.exp: Add new tests.
diff --git a/gdb/testsuite/gdb.base/batch-preserve-term-settings.exp b/gdb/testsuite/gdb.base/batch-preserve-term-settings.exp
index 7ae3518..c39d638 100644
--- a/gdb/testsuite/gdb.base/batch-preserve-term-settings.exp
+++ b/gdb/testsuite/gdb.base/batch-preserve-term-settings.exp
@@ -28,12 +28,13 @@ if [is_remote host] {
 }
 
 # The shell's prompt.
-set shell_prompt_re "\[$#\] "
+set shell_prompt_ps1 "gdb-subshell$ "
+set shell_prompt_re [string_to_regexp $shell_prompt_ps1]
 
 # Spawn shell.  Returns true on success, false otherwise.
 
 proc spawn_shell {} {
-    global shell_prompt_re
+    global shell_prompt_ps1 shell_prompt_re
 
     set res [remote_spawn host "/bin/sh"]
     if { $res < 0 || $res == "" } {
@@ -41,10 +42,12 @@ proc spawn_shell {} {
 	return 0
     }
 
+    send_gdb "PS1=\"$shell_prompt_ps1\"\n"
+
     set gotit 0
     set test "spawn shell"
     gdb_expect {
-	-re "$shell_prompt_re$" {
+	-re "PS1=\[^\r\n\]*\r\n$shell_prompt_re$" {
 	    pass $test
 	    set gotit 1
 	}