Suppress all -Wmissing-field-initializers in the test suite

There were many dozens of such warnings given for the various arrays
used in the tests and it just doesn't seem feasible nor desirable to fix
them all, so just suppress the warnings for all these arrays to get rid
of them with recent (7+?) versions of gcc.

tests/strings/strings.cpp

@@ -612,6 +612,8 @@ typedef wxLongLong_t TestValue_t;
 typedef long TestValue_t;
 #endif
 
+wxGCC_WARNING_SUPPRESS(missing-field-initializers)
+
 static const struct ToLongData
 {
     const wxChar *str;
@@ -667,6 +669,8 @@ static const struct ToLongData
     { wxT("0x11"), 17, Number_Ok,      16 },
 };
 
+wxGCC_WARNING_RESTORE(missing-field-initializers)
+
 void StringTestCase::ToLong()
 {
     long l;