[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.