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.

gdb/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2020-11-17  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* gdb.gdb/unittest.exp: Spot 'Running...' lines.
+
 2020-11-17  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* gdb.base/completion.exp: Add new tests.

gdb/testsuite/gdb.gdb/unittest.exp

@@ -26,6 +26,13 @@ gdb_start
 
 set test "maintenance selftest"
 gdb_test_multiple $test $test {
+  -re ".*Running selftest \[^\n\r\]+\." {
+	# The selftests can take some time to complete.  To prevent
+	# timeout spot the 'Running ...' lines going past, so long as
+	# these are produced quickly enough then the overall test will
+	# not timeout.
+	exp_continue
+  }
   -re "Ran ($decimal) unit tests, 0 failed\r\n$gdb_prompt $" {
 	set num_ran $expect_out(1,string)
 	gdb_assert "$num_ran > 0" $test