tests: Verify ExpressionMatch test data won't OOB

Verify that the ExpressionMatch test data for test
trimMultiWildcardWhitespace won't try to access out of bounds data by
changing EXPECT to ASSERT for patternPair.size() == 2.

If the test data is wrong, the results will be undefined, so there's
no point in continuing the test.

Alternative: use std::vector's .at() function, which will raise a
runtime error if out of bounds.