tests: Remove use of wfRandomString() for test fixtures

tests: Remove use of wfRandomString() for test fixtures

This reduces confidence in the test. There is no guruantee that
it won't return the same value twice during the duration of a full
PHPUnit run of all test suites, whether twice in a row or 20 minutes
apart.

For a test that needs a string of any kind, use an explicit, consinstent
and cheap literal value.

For a test that specifically needs some kind of uniqueness compared to
something else within the same test case, do so explicitly.

Tests that require something globally unique (for some undefined/vague
definition of "global") were not found, and should not exist anyway.

Also, in libs/objectcache tests, fix order of parameters in some
assertions (expected first, then actual), and use assertFalse/assertSame
instead of assertEqual for cases where false is expected to remove
tolerance of other loosely equal values.

Change-Id: Ifc60e88178da471330b94bfbf12e2731d2efc77d