MWCryptHKDFTest: Mock $wgSecretKey to ensure atomicity

Follows-up af66c04.

The tests depend indirectly on wgSecretKey (used by
MWCryptHKDF::generateHex). On test installs I tend to set this
to "example" or "***", which is less than 16 characters and thus
the unit tests failed with "MWCryptHKDF secret was too short".

Change-Id: I9a85fd9fbb345863e4837fed2910d60ebf1b0d83
Merge "MWCryptHKDFTest: Mock $wgSecretKey to ensure atomicity"