Build and run isl_test as part of check-polly

Running isl tests is important to gain confidence that the isl build we created
works as expected. Besides the actual isl tests, there are also isl AST
generation tests shipped with isl. This change only adds support for the isl
unit tests. AST generation test support is left for a later commit.

There is a choice to run tests directly through the build system or in the
context of lit. We choose to run tests as part of lit to as this allows us to
easily set environment variables, print output only on error and generally run
the tests directly from the lit command.

Reviewers: brad.king, Meinersbur

Subscribers: modocache, brad.king, pollydev, beanz, llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D25155

llvm-svn: 283245