Use TemplateHaskell to decorate tests with names

This makes error message change from "Test 4 failed …" to "Test
prop_Loader_mergeData failed", which is much more readable. It also
removes the duplication of test suite names in the test.hs file.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>