Fix invocation with explicit .datax file

Don't chdir when invoking a test suite executable with an explicit .datax
file. The point of the chdir is to automatically find the .datax file (and
the relative location of the data_files directory) in typical cases. This
conflicts with the expectation that passing a relative path to a .datax file
will work.

(This is what I had originally intended, and what is documented in the
comment, but I forgot to add the argc check in the initial commit.)

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

tests/suites/main_test.function

@@ -248,7 +248,9 @@ int main(int argc, const char *argv[])
      * test-specific files such as the outcome file, which is arguably
      * not desirable and should be fixed later.
      */
-    try_chdir_if_supported(argv[0]);
+    if (argc == 1) {
+        try_chdir_if_supported(argv[0]);
+    }
 
     int ret = mbedtls_test_platform_setup();
     if (ret != 0) {