Tweak #5202

Back out the new `-build-only` for tests in favor of the more established `-build-mode:test`, but retain the new `-keep-test-executable` option and default cleanup of test executables.

src/build_settings.cpp

@@ -441,7 +441,6 @@ struct BuildContext {
 	String extra_assembler_flags;
 	String microarch;
 	BuildModeKind build_mode;
-	bool   build_only;
 	bool   keep_test_executable;
 	bool   generate_docs;
 	bool   custom_optimization_level;