Test bind exceptions

Add a few fixes that avoid having multiple `SparkContext`s contending for the default UI port 4040, which was causing spurious socket-bind exceptions.

In one specific case, `SomaticThresholdVariantCallerSuite`, two `SparkContext`s were being created during test's execution. In general there were some races where parallel test suites would run into this issue.

gist of previous test output including spurious exceptions: https://gist.github.com/ryan-williams/1b4f248d40f95fdd0292

gist of new test output without spurious exceptions: https://gist.github.com/ryan-williams/9567235e079e0cbdd2c8