tests: Restore sleep for ninja on HFS+ (macOS)

QuLogic discovered that HFS+ only stores dates in uint32 seconds since
the epoch, so ninja cannot report sub-1s resolution timestamps there.
Sometime in the future Apple FS will become widely-available and we
will have to add a filesystem check at startup.

https://developer.apple.com/legacy/library/technotes/tn/tn1150.html#HFSPlusDates

run_tests.py

@@ -105,7 +105,10 @@ def get_backend_commands(backend, debug=False):
 
 def ensure_backend_detects_changes(backend):
     # We're using a ninja with QuLogic's patch for sub-1s resolution timestamps
-    if 'MESON_FIXED_NINJA' in os.environ:
+    # and not running on HFS+ which only stores dates in seconds:
+    # https://developer.apple.com/legacy/library/technotes/tn/tn1150.html#HFSPlusDates
+    # FIXME: Upgrade Travis image to Apple FS when that becomes available
+    if 'MESON_FIXED_NINJA' in os.environ and not mesonlib.is_osx():
         return
     # This is needed to increase the difference between build.ninja's
     # timestamp and the timestamp of whatever you changed due to a Ninja