Expand unit tests for fuzzer::Merger

This change adds additional unit tests for fuzzer::Merger::Parse and fuzzer::Merger::Merge in anticipation of additional changes to the merge control file format to support cross-process fuzzing.

It modifies the parameter handling of Merge slightly in order to make NewFeatures and NewCov consistent with NewFiles; namely, Merge *replaces* the contents of these output parameters rather than accumulating them (thereby fixing a buggy return value).

This is change 1 of (at least) 18 for cross-process fuzzing support.

Reviewed By: morehouse

Differential Revision: https://reviews.llvm.org/D94506