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"

2 files changed, 11 insertions, 3 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index f38efbe..9129c11 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2019-07-03  Pedro Alves  <palves@redhat.com>
+
+	* gdb.base/settings.exp (test-string): Adjust expected out when
+	testing "maint test-settings show filename"
+
 2019-07-02  Pedro Alves  <palves@redhat.com>
 
 	* gdb.base/options.exp (test-info-threads): New procedure.
diff --git a/gdb/testsuite/gdb.base/settings.exp b/gdb/testsuite/gdb.base/settings.exp
index aeca67c..b691ad8 100644
--- a/gdb/testsuite/gdb.base/settings.exp
+++ b/gdb/testsuite/gdb.base/settings.exp
@@ -447,9 +447,12 @@ proc test-string {variant} {
     set set_cmd "maint test-settings set $variant"
     set show_cmd "maint test-settings show $variant"
 
-    # Empty string.  Also checks that gdb doesn't crash if we haven't
-    # set the string yet.
-    gdb_test "$show_cmd" "^$show_cmd\r\n" "$show_cmd: empty first time"
+    # Checks that gdb doesn't crash if we haven't set the string yet.
+    if {$variant != "filename"} {
+	gdb_test "$show_cmd" "^$show_cmd\r\n" "$show_cmd: show default"
+    } else {
+	gdb_test "$show_cmd" "/foo/bar" "$show_cmd: show default"
+    }
 
     # A string value.
     gdb_test_no_output "$set_cmd hello world"