CDRIVER-732: Allow passing test arguments to debug test runs

We still want to make sure we don't fork & run them in parallel, so
those arguments are hardcoded, but we now support passing for example
the test name/prefix (-l).

For example:

    $ TEST_ARGS="-l /Uri/*" make debug

Author: bjori

tests/Makefile.am

@@ -228,4 +228,4 @@ DEBUGGER = gdb --args
 endif
 
 debug: test-libmongoc
-	$(LIBTOOL) --mode=execute $(DEBUGGER) test-libmongoc -f -p
+	$(LIBTOOL) --mode=execute $(DEBUGGER) test-libmongoc $(TEST_ARGS) -f -p