test: show error message if $SYSTEMD_TEST_DATA does not exist

Rename get_exe_relative_testdata_dir() to get_testdata_dir() and move
the env var check into that, so that everything interesting happens at
the same place.

src/shared/tests.c

@@ -36,11 +36,22 @@ char* setup_fake_runtime_dir(void) {
         return p;
 }
 
-const char* get_exe_relative_testdata_dir(void) {
+const char* get_testdata_dir(void) {
+        const char *env;
         _cleanup_free_ char *exedir = NULL;
         /* convenience: caller does not need to free result */
         static char testdir[PATH_MAX];
 
+        /* if the env var is set, use that */
+        env = getenv("SYSTEMD_TEST_DATA");
+        if (env) {
+                if (access(env, F_OK) >= 0)
+                        return env;
+
+                fputs("ERROR: $SYSTEMD_TEST_DATA directory does not exist\n", stderr);
+                exit(1);
+        }
+
         assert_se(readlink_and_make_absolute("/proc/self/exe", &exedir) >= 0);
 
         /* Check if we're running from the builddir. If so, use the compiled in path. */

src/shared/tests.h

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

src/test/test-helper.h

@@ -42,5 +42,5 @@
                -ENOMEDIUM /* cannot determine cgroup */         \
                )
 
-#define TEST_DATA_DIR(subdir)                                     \
-        strjoina(getenv("SYSTEMD_TEST_DATA") ?: get_exe_relative_testdata_dir(), subdir)
+#define TEST_DATA_DIR(subdir)                                   \
+        strjoina(get_testdata_dir(), subdir)