Do not fetch test data for CI runs

Since the unit test data is now included in the main repository, we do not
need to fetch this for the CI jobs which was a frequent source of spurious
build failures.

This also sets up per-job caches so that GCC and Clang builds do not share
a cache and hence the cache that has to be fetched for each job is smaller.
3 jobs from include-unit-test-data in 4 minutes and 31 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #24731
build

00:04:31

passed #24732
build_clang_libcpp

00:04:16

passed #24733
build_gcc_49

00:02:52