gdb/testsuite: fix race in gdb.threads/signal-while-stepping-over-bp-other-thread.exp

Commit 3ec3145c5d ("gdb: introduce scoped debug prints") updated some
tests using "set debug infrun" to handle the fact that a debug print is
now shown after the prompt, after an inferior stop.  The same issue
happens in gdb.threads/signal-while-stepping-over-bp-other-thread.exp.
If I run it in a loop, it eventually fails like these other tests.

The problem is that the testsuite expects to see $gdb_prompt followed by
the end of the buffer.  It happens that expect reads $gdb_prompt and the
debug print at the same time, in which case the regexp never matches and
we get a timeout.

The fix is the same as was done in 3ec3145c5d, make the testsuite
believe that the prompt is the standard GDB prompt followed by that
debug print.

Since that test uses gdb_test_sequence, and the expected prompt is in
gdb_test_sequence, add a -prompt switch to gdb_test_sequence to override
the prompt used for that call.

gdb/testsuite/ChangeLog:

	* lib/gdb.exp (gdb_test_sequence): Accept -prompt switch.
	* gdb.threads/signal-while-stepping-over-bp-other-thread.exp:
	Pass prompt containing debug print to gdb_test_sequence.

Change-Id: I33161c53ddab45cdfeadfd50b964f8dc3caa9729

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2021-01-06  Simon Marchi  <simon.marchi@polymtl.ca>
+
+	* lib/gdb.exp (gdb_test_sequence): Accept -prompt switch.
+	* gdb.threads/signal-while-stepping-over-bp-other-thread.exp:
+	Pass prompt containing debug print to gdb_test_sequence.
+
 2021-01-04  Tom de Vries  <tdevries@suse.de>
 
 	* gdb.dwarf2/dw2-out-of-range-end-of-seq.exp: New file.

gdb/testsuite/gdb.threads/signal-while-stepping-over-bp-other-thread.exp

@@ -102,7 +102,7 @@ gdb_test "set scheduler-locking off"
 gdb_test "set debug infrun 1"
 
 set test "step"
-gdb_test_sequence $test $test {
+gdb_test_sequence $test $test -prompt "$gdb_prompt \\\[infrun\\\] fetch_inferior_event: exit\r\n$"  {
     "need to step-over"
     "resume_1: step=1,"
     "signal arrived while stepping over breakpoint"

gdb/testsuite/lib/gdb.exp

@@ -1401,6 +1401,9 @@ proc gdb_test_no_output { args } {
 # EXPECTED_OUTPUT_LIST is a list of regexps of expected output, which are
 # processed in order, and all must be present in the output.
 #
+# The -prompt switch can be used to override the prompt expected at the end of
+# the output sequence.
+#
 # It is unnecessary to specify ".*" at the beginning or end of any regexp,
 # there is an implicit ".*" between each element of EXPECTED_OUTPUT_LIST.
 # There is also an implicit ".*" between the last regexp and the gdb prompt.
@@ -1413,16 +1416,32 @@ proc gdb_test_no_output { args } {
 #    0 if the test passes,
 #   -1 if there was an internal error.
 
-proc gdb_test_sequence { command test_name expected_output_list } {
+proc gdb_test_sequence { args } {
     global gdb_prompt
+
+    parse_args {{prompt ""}}
+
+    if { $prompt == "" } {
+	set prompt "$gdb_prompt $"
+    }
+
+    if { [llength $args] != 3 } {
+	error "Unexpected # of arguments, expecting: COMMAND TEST_NAME EXPECTED_OUTPUT_LIST"
+    }
+
+    lassign $args command test_name expected_output_list
+
     if { $test_name == "" } {
 	set test_name $command
     }
+
     lappend expected_output_list ""; # implicit ".*" before gdb prompt
+
     if { $command != "" } {
 	send_gdb "$command\n"
     }
-    return [gdb_expect_list $test_name "$gdb_prompt $" $expected_output_list]
+
+    return [gdb_expect_list $test_name $prompt $expected_output_list]
 }