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:
2019-07-03  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:
2019-07-03  Pedro Alves  <palves@redhat.com>

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

1 files changed, 11 insertions, 0 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index d50fe89..05c25b4 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,14 @@
+2019-07-03  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.
+
 2019-07-02  Simon Marchi  <simon.marchi@polymtl.ca>
 
 	* breakpoint.h (remove_breakpoints_inf): Change return type to