Convert TestClasspath to test fixtures

The main complication with this change was that many of the tests check
paths that are provided by the default java modules. The location of
them is different in the test fixtures that it is when using
testConfig() and so the test needed to be changed.

Bug: 182638834
Test: m nothing
Change-Id: I6d325dbd3ba39e5de7e53c576d7cfe07bd95a965