mock 'time()' in cache tests

instead of calling 'sleep()' to let time advance.

This shortens the time needed to run those tests,
and ensures consistent results.
(Tests would randomly fail when using 'sleep()')

gallery_dl/cache.py

@@ -57,7 +57,7 @@ class MemoryCacheDecorator(CacheDecorator):
             value, expires = self.cache[key]
         except KeyError:
             expires = 0
-        if expires < timestamp:
+        if expires <= timestamp:
             value = self.func(*args, **kwargs)
             expires = timestamp + self.maxage
             self.cache[key] = value, expires