Ensure test coverage for spilling disabled for all spilling operators

We should ensure that we have test coverage for disabling spilling. There are two dimensions. I don't think we need coverage of the whole matrix, but we need coverage along both dimensions.

Query operators:

• Agg - covered by TestScratchLimit
• Hash join - covered by TestScratchLimit (and with in-review IMPALA-5570 patch)
• Sort - covered by TestScratchLimit and TestScratchDir
• Analytic - covered by TestScratchLimit

Means of disabling:

• scratch_limit = 0 - covered by TestScratchLimit
• disable_unsafe_spills = true + a table missing stats - NOT COVERED
• starting up with no scratch disks - covered by TestScratchDir