phpunit: Make LogFormatterTest less slow

phpunit: Make LogFormatterTest less slow

Right now it's quite slow because it reloads LocalisationCache
for every test case.

Change the reload to only happen once for the entire class.

Also use RequestContext instead of wgLang to match logic in
the parent class (MediaWikiLangTestCase).

$ phpunit includes/logging/LogFormatterTest.php
Before
- run 1) Time: 23.56 seconds, Memory: 48.00MB
- run 2) Time: 22.41 seconds, Memory: 48.75MB
After
- run 1) Time: 14.27 seconds, Memory: 47.50MB
- run 2) Time: 13.45 seconds, Memory: 47.25MB

Change-Id: Ia791563ca9094c36782acfe9826def45f2a1d621