[gdb/testsuite] Fix error handling in gdb_file_cmd

Consider a gdb_load patch to call the gdb_file_cmd twice:
...
 proc gdb_load { arg } {
     if { $arg != "" } {
+       set res [gdb_file_cmd $arg]
+       if { $res != 0 } {
+           return $res
+       }
        return [gdb_file_cmd $arg]
     }
     return 0
 }
...

When running test-case gdb.base/index-cache.exp, we run into:
...
ERROR: Couldn't load outputs/gdb.base/index-cache/index-cache, other program \
  already loaded (timeout).
FAIL: gdb.base/index-cache.exp: test_cache_enabled_miss: check index-cache \
  stats (GDB internal error)
ERROR: Couldn't load outputs/gdb.base/index-cache/index-cache, other program \
  already loaded (timeout).
...

The first timeout in more detail:
...
(gdb) file outputs/gdb.base/index-cache/index-cache^M
Load new symbol table from "index-cache"? (y or n) y^M
Reading symbols from index-cache...^M
src/gdb/dwarf2/read.c:2540: internal-error: \
  void create_cus_from_index(dwarf2_per_bfd*, const gdb_byte*, offset_type, \
                             const gdb_byte*, offset_type): \
  Assertion `per_bfd->all_comp_units.empty ()' failed.^M
A problem internal to GDB has been detected,^M
further debugging may prove unreliable.^M
Quit this debugging session? (y or n) ERROR: Couldn't load index-cache, \
  other program already loaded (timeout).
...

Proc gdb_file_cmd has a gdb_expect handling the result of the file command,
and if the result is a "Load new symbol table from index-cache? (y or n) "
prompt, it sends a "y" and enters in a nested gdb_expect to handle the
result.

The first gdb_expect contains code to handle "A problem internal to GDB has
been detected", but the second one doesn't, which causes the timeout.

Fix this by removing the nested gdb_expect, and using exp_continue instead,
such that we have instead:
...
ERROR: Couldn't load outputs/gdb.base/index-cache/index-cache -- with new \
  symbol table into gdb (GDB internal error).
ERROR: Couldn't load outputs/gdb.base/index-cache/index-cache -- with new \
  symbol table into gdb (GDB internal error).
...

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-06-04  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (gdb_file_cmd): Replace incomplete gdb_expect by
	exp_continue.

2 files changed, 13 insertions, 15 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index ab9443e..9d97938 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,10 @@
 2020-06-04  Tom de Vries  <tdevries@suse.de>
 
+	* lib/gdb.exp (gdb_file_cmd): Replace incomplete gdb_expect by
+	exp_continue.
+
+2020-06-04  Tom de Vries  <tdevries@suse.de>
+
 	* lib/gdb.exp (gdb_file_cmd): Use perror instead of fail.
 
 2020-06-03  Tom de Vries  <tdevries@suse.de>
diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index 63a9e3d..3cdaefa 100644
--- a/gdb/testsuite/lib/gdb.exp
+++ b/gdb/testsuite/lib/gdb.exp
@@ -1761,6 +1761,7 @@ proc gdb_file_cmd { arg } {
     }
 
     send_gdb "file $arg\n"
+    set new_symbol_table 0
     gdb_expect 120 {
 	-re "Reading symbols from.*LZMA support was disabled.*$gdb_prompt $" {
 	    verbose "\t\tLoaded $arg into $GDB; .gnu_debugdata found but no LZMA available"
@@ -1778,22 +1779,14 @@ proc gdb_file_cmd { arg } {
 	    return 0
         }
         -re "Load new symbol table from \".*\".*y or n. $" {
+	    if { $new_symbol_table > 0 } {
+		perror "Couldn't load $arg, interactive prompt loop detected."
+		return -1
+	    }
             send_gdb "y\n" answer
-            gdb_expect 120 {
-                -re "Reading symbols from.*$gdb_prompt $" {
-                    verbose "\t\tLoaded $arg with new symbol table into $GDB"
-		    set gdb_file_cmd_debug_info "debug"
-		    return 0
-                }
-                timeout {
-                    perror "Couldn't load $arg, other program already loaded (timeout)."
-		    return -1
-                }
-		eof {
-		    perror "Couldn't load $arg, other program already loaded (eof)."
-		    return -1
-		}
-            }
+	    incr new_symbol_table
+	    set arg "$arg -- with new symbol table"
+	    exp_continue
 	}
         -re "No such file or directory.*$gdb_prompt $" {
             perror "($arg) No such file or directory"