In test/bench.py, added "internal" flag

This marks internal tests/benches (case.in="lfs.c") with an otherwise-unused
flag that is printed during --summary/--list-*. This just helps identify which
tests/benches are internal.

runners/test_runner.h

@@ -33,7 +33,8 @@ void test_trace(const char *fmt, ...);
 struct lfs_config;
 
 enum test_flags {
-    TEST_REENTRANT = 0x1,
+    TEST_INTERNAL  = 0x1,
+    TEST_REENTRANT = 0x2,
 };
 typedef uint8_t test_flags_t;