gdb/testsuite: prevent timeout in gdb.gdb/unittest.exp

gdb/testsuite: prevent timeout in gdb.gdb/unittest.exp

When GDB is compiled with --enable-targets=all I would sometimes see
the 'maintenance selftest' in gdb.gdb/unittest.exp test timeout.

This one command causes GDB to run many separate self tests, this can
take some time.  The output of this command basically follows this
pattern:

  (gdb) maintenance selftest
  Running selftest aarch64-analyze-prologue.
  Running selftest aarch64-process-record.
  Running selftest arm-record.
  Running selftest arm_analyze_prologue.
  Running selftest array_view.
  Running selftest child_path.
  Running selftest cli_utils.
  ..... snip lots more lines ....
  Ran 79 unit tests, 0 failed

Currently the expect script waits for the final summary line ("Ran 79
unit test, 0 failed") before declaring pass or fail.  The problem is
that if the summary line takes too long to appear the test will
timeout.

As this test makes use of gdb_test_multiple then all I've done is add
an extra pattern that matches the 'Running selftest ....' lines and
then calls exp_continue.  Doing this means we find matches much more
frequently, and each time we do the timeout timer resets, preventing
the overall test from timing out.

gdb/testsuite/ChangeLog:

	* gdb.gdb/unittest.exp: Spot 'Running...' lines.