Fix so wfResetOutputBuffers doesn't break unit tests

Fix so wfResetOutputBuffers doesn't break unit tests

If something being unit tested calls wfResetOutputBuffers(), it'll break
PHPUnit's attempt to capture output in order to fail tests. In the old
version we're currently using for Jenkins it will throw a warning
"ob_end_clean(): failed to delete buffer. No buffer to delete", while in
newer versions it'll detect that its buffer was removed and fail the
test as risky instead.

The solution here is to have MediaWikiTestCase add a buffer with a known
name in setUp() (and remove it in tearDown()), and have
wfResetOutputBuffers() stop when it sees that known name on the stack.

Bug: T109843
Change-Id: I8acd91ec9dd9c6d78a5d91d96202249f571d5d83