lib/drmtest: __ prefix for drmtest_run_subtest

Now that callers must setup the longjmp target (we can't do that in
run_subtest itself since that callframe won't survive) tests shouldn't
call this function directly any more.

Make this clear by adding a __ prefix.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

lib/drmtest.c

@@ -717,7 +717,7 @@ out:
  * outside of places protected by drmtest_run_subtest checks - the piglit
  * runner adds every line to the subtest list.
  */
-bool drmtest_run_subtest(const char *subtest_name)
+bool __drmtest_run_subtest(const char *subtest_name)
 {
 	assert(in_subtest == false);