Use multiple test scripts, one for each subdirectory containing test
files, instead of having one test script to manage them all.

This allows removing some workarounds, such as the need to create
symlinks in the test run directory.

gcc/testsuite/ChangeLog:

	* gdc.test/compilable/compilable.exp: New file.
	* gdc.test/fail_compilation/fail_compilation.exp: New file.
	* gdc.test/gdc-test.exp: Remove.
	* gdc.test/runnable/runnable.exp: New file.
	* lib/gdc-utils.exp: ...this.  Remove load_lib gdc-dg.exp.
	(dmd2dg): Rename to...
	(gdc-convert-test): ...this.
	(gdc-do-test): Add testcases parameter and remove subdir handling.