IntegrationTester.prepEval and improved WatchTests

[arturaz]

While debugging com-lihaoyi/os-lib#398 I encountered a lackluster developer experience when running WatchTests.

• It took ages for the tests to fail.
• When the tests failed, the amount of context was very low.

This PR improves on both of those areas:

• WatchTests timeout is now 15s instead of 120s when running not on CI.
• If the test fails the eval context (things like working directory, the command line for eval and environment variables) is printed out.

To achieve that, I introduced:

• prepEval - as eval but doesn't actually execute immediatelly.
• withTestClues - enhances utest.AssertionError thrown in the block with the given TestValues