Tidy-up tests, introduce LoadingTest.TestApparatus & TestFetching.withStubDataStore

[rtyley]

These improvements are partially motivated by the changes in this PR:

• Support fetching-then-parsing-with-key #110

...as that PR adds a fetching.thenParsingWithKey() method to complement fetching.thenParsing(), it's good to tidy up the test for thenParsing() first, to make the subsequent introduction of testing on the sister-method thenParsingWithKey() method nice & small.

Changes

• Create the com.gu.etagcaching.testkit package to contain useful functionality for tests
  • CountingParser - a parser that counts how many times it's used, so we can test we're not doing any unnecessary parsing
  • TestFetching moves into the testkit package, and gains TestFetching.withStubDataStore to let us create stub Fetching instances that will return whatever data we choose.
• LoadingTest.TestApparatus added, to keep the checks for unnecessary-parsing concise.

[rtyley]

This test ("provide callbacks that allow logging updates") was previously testing a bit more than just the target functionality, so it was more complex than it needed to be.

Switching to a FreshnessPolicy of AlwaysWaitForRefreshedValue (rather than TolerateOldValueWhileRefreshing) simplifies things because it means we always will receive the latest value, and don't need to add in artificial delays in order to get the right value.