[gdb/testsuite] Fix read1 timeout in gdb.base/gdb-sigterm.exp

When running check-read1, I run into a timeout in test-case
gdb.base/gdb-sigterm.exp:
...
  [infrun] handle_inferior_event: status->kind = stopped, \
    signal = GDB_SIGNAL_TRAP^M
  [infrun] start_step_over: enter^M
    [infrun] start_step_overFAIL: gdb.base/gdb-sigterm.exp: \
      expect eof #0 (timeout)
gdb.base/gdb-sigterm.exp: expect eof #0: stepped 0 times
FAIL: gdb.base/gdb-sigterm.exp: 50 SIGTERM passes
...

The corresponding gdb_test_multiple has an exp_continue clause, but it doesn't
trigger because the regexp greps for 'infrun: ' instead of '[infrun] '.

Fix the timeout by fixing the infrun regexp.

Tested on x86_64-linux, with check and check-read1.

gdb/testsuite/ChangeLog:

2021-05-11  Tom de Vries  <tdevries@suse.de>

	* gdb.base/gdb-sigterm.exp: Fix exp_continue regexp.

gdb/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2021-05-11  Tom de Vries  <tdevries@suse.de>
+
+	* gdb.base/gdb-sigterm.exp: Fix exp_continue regexp.
+
 2021-05-11  Bhuvanendra Kumar  <Bhuvanendra.KumarN@amd.com>
 
 	* gdb.fortran/array-element.exp: Breakpoint location is modified.

gdb/testsuite/gdb.base/gdb-sigterm.exp

@@ -78,7 +78,7 @@ proc do_test { pass } {
 	    verbose -log "$pf_prefix $test: got eof"
 	    set abort 0
 	}
-	-re "infrun: process_event_stop_test: stepping inside range" {
+	-re {\[infrun\] process_event_stop_test: stepping inside range} {
 	    incr stepping
 	    exp_continue
 	}