tests/run-tests: Auto-skip extmod/ticks_diff, extmod/time_ms_us tests.

tests/extmod/ticks_diff.py

@@ -1,4 +1,8 @@
-from utime import ticks_diff, ticks_add
+try:
+    from utime import ticks_diff, ticks_add
+except ImportError:
+    print("SKIP")
+    raise SystemExit
 
 MAX = ticks_add(0, -1)
 # Should be done like this to avoid small int overflow

tests/extmod/time_ms_us.py

@@ -1,7 +1,7 @@
-import utime
 try:
-    utime.sleep_ms
-except AttributeError:
+    import utime
+    utime.sleep_ms, utime.sleep_us, utime.ticks_diff, utime.ticks_ms, utime.ticks_us, utime.ticks_cpu
+except (ImportError, AttributeError):
     print("SKIP")
     raise SystemExit