Fix defaults of some "maint test-settings" subcommands

Fix defaults of some "maint test-settings" subcommands

New tests added later for the incoming "with" command exposed a couple
invalid-default-value bugs in the "maint test-settings" commands:

- var_filename commands don't allow setting the filename to the empty
  string (unlike var_optional_filename commands), yet, "maint
  test-settings filename"'s control variable was not initialized, so
  on startup, "maint test-settings show filename" shows an empty
  string.

- "maint test-settings enum"'s control variable was not initialized,
  so on startup, "maint test-settings show enum" shows an empty value
  instead of a valid enum value.

Both issues are fixed by initializing the control variables.

gdb/ChangeLog:
yyyy-mm-dd  Pedro Alves  <palves@redhat.com>

	* maint-test-settings.c (maintenance_test_settings_xxx)
	(maintenance_test_settings_yyy, maintenance_test_settings_zzz):
	New.
	(maintenance_test_settings_enums): Use them.
	(maintenance_test_settings_enum): Default to
	maintenance_test_settings_xxx.
	(_initialize_maint_test_settings): Initialize
	MAINTENANCE_TEST_SETTINGS_FILENAME.

gdb/testsuite/ChangeLog:
yyyy-mm-dd  Pedro Alves  <palves@redhat.com>

	* gdb.base/settings.exp (test-string): Adjust expected out when
	testing "maint test-settings show filename"