Ensure subtest failures are reported on subtest

A subtest (code inside the func passed to t.Run(...)) are passed their
own t *Testing pointer to use to report errors in order to ensure that
they are treated as errors of the subtest and not the containing test.

This change ensures that the subtests in the following tests use the
correct t *Testing.
* TestPrebuiltExportDexImplementationJars
* TestBootDexJarsFromSourcesAndPrebuilts

Bug: 178361284
Test: m nothing
Change-Id: I4e8b166051cb6098c89d8e68a450c81a714f7677