test: drop TEST_DATA_DIR, fold into get_testdata_dir()

Drop the TEST_DATA_DIR macro as this was using alloca() within a
function call which is allegedly unsafe. So add a "suffix" argument to
get_testdata_dir() instead and call that directly.

src/shared/tests.h

@@ -20,4 +20,4 @@
 ***/
 
 char* setup_fake_runtime_dir(void);
-const char* get_testdata_dir(void);
+const char* get_testdata_dir(const char *suffix);