Fix getGriefingCollateral tests

[bvotteler]

Adding multi-collateral vaults to the tests have broken “should getGriefingCollateral” tests for both issue and replace.

It seems the running oracle is interfering with the artificially set exchange rate.

To fix: Look at oracle tests where we use mocks to fix the exchange rate for the scope of the test and re-use that pattern.

Additional note: Look at Greg’s comment here, and check if these tests can be reworked completely

[gregdhill]

Closed by #578