search/searcher: fix dropped test errors

[alrs]

This fixes three dropped err variables in the tests for search/searcher.

The missing err in TestBooleanSearch() was simply omitted. The mishandled err variables in testScorchCompositeSearchOptimizationsHelper() and TestTermRangeSearchTooManyTerms() were being redefined with a :-= inside a for loop, so both were lost before the if err != nil test outside the loop.