testsuite: Handle more kinds of gdc.test test flags and directives.

testsuite: Handle more kinds of gdc.test test flags and directives.

This patch adds cases for JSON and D header file generation flags in the
testsuite, and sets up the test accordingly to only compile, then check
that the expected output file exists.

Support has also been added for the DFLAGS test directive, which
disables testing with all default flag permutations.

gcc/testsuite/ChangeLog:

	* lib/gdc-utils.exp: (gdc-convert-args): Handle compilation test
	arguments -D, -H, -X, and set compilable_output_file_ext.
	(gdc-convert-test): Document and add cases DFLAGS and
	EXTRA_CPP_SOURCES.  Add dg-final entry for output generation tests,
	and set compilable_do_what to compile.
	(gdc-do-test): Define compilable_do_what, default to assemble.