Unit tests for *_metadata_by_mid

[kovshenin]

Let's create a few tests for the get, update and delete _metadata_by_mid() functions.

Here's a patch for the update_metadata_by_mid function test.

Howdy all, this is my first time at unit testing for WordPress?, so go easy on me. I've created a short test for the update_metadata_by_mid() function which has a patch in this ticket ​http://core.trac.wordpress.org/ticket/18195

A local run went smooth with 5 assertions. Hoping for some feedback!

Improved version of the test case

I've attached an improved version of the test case, moved all the initialization to the setup method, added some invalid keys and meta id runs. Cleanup after usage. Hopefully this will do better.

Testing for cached values

Third patch runs a test to see if caches got cleared after a meta update. This works with the latest patch to the function itself over at ​#wp18195. Let me know how this works for you. Thanks!

Tests both update and delete _metadata_by_mid functions

Here's a version that tests both update and delete functions. The patch for both functions together has been uploaded here ​#wp18195. Have a good day!

(In [384]) Update and delete by mid tests. Props kovshenin. fixes #21