[cmake] Add polly-isl-test dependency to lit tests.

lit recursively iterates through the test subdirectories and finds the ISL
unittest. For this test to work, the polly-isl-test executable needs to be
compiled.

Add the polly-isl-test dependency to POLLY_TEST_DEPS. This makes check-polly and
check-polly-tests work from a fresh build directory.

llvm-svn: 284339