Fix fuzz variable mask test

This jobs has been both broken and not even running for some time. A few changes were required to make it run and pass.

• Move fuzz test file and its corpus test data to a package where they can run. The test attempted to tests code in an internal package it could not access.

• Fix some of the types in the test to reflect the change in the production code

• Workaround build failure: go-fuzz-build fails with the following error:

    go-fuzz-build -libfuzzer -o fuzz_variable_mask.a ./common/buildlogger/internal
    failed to execute go build: exit status 1
    # crypto/internal/bigmod
    /usr/local/go/src/crypto/internal/bigmod/nat_asm.go:21: misplaced compiler directive

This is a known issue (https://github.com/dvyukov/go-fuzz/issues/354). The discussion in the issue mentions on a workaround, with was implemented here.