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/bench_runner.h

@@ -40,7 +40,7 @@ void bench_stop(void);
 struct lfs_config;
 
 enum bench_flags {
-    BENCH_REENTRANT = 0x1,
+    BENCH_INTERNAL  = 0x1,
 };
 typedef uint8_t bench_flags_t;