Add a vendor dependency used for tests

vendor/github.com/google/go-cmp/LICENSE

vendor/github.com/google/go-cmp/cmp/compare.go

vendor/github.com/google/go-cmp/cmp/export_panic.go

vendor/github.com/google/go-cmp/cmp/export_unsafe.go

...thub.com/google/go-cmp/cmp/internal/diff/debug_disable.go

...ithub.com/google/go-cmp/cmp/internal/diff/debug_enable.go

vendor/github.com/google/go-cmp/cmp/internal/diff/diff.go

vendor/github.com/google/go-cmp/cmp/internal/flags/flags.go

....com/google/go-cmp/cmp/internal/flags/toolchain_legacy.go

....com/google/go-cmp/cmp/internal/flags/toolchain_recent.go

...or/github.com/google/go-cmp/cmp/internal/function/func.go

...ub.com/google/go-cmp/cmp/internal/value/pointer_purego.go

...ub.com/google/go-cmp/cmp/internal/value/pointer_unsafe.go

vendor/github.com/google/go-cmp/cmp/internal/value/sort.go

vendor/github.com/google/go-cmp/cmp/internal/value/zero.go

vendor/github.com/google/go-cmp/cmp/options.go

vendor/github.com/google/go-cmp/cmp/path.go

vendor/github.com/google/go-cmp/cmp/report.go

vendor/github.com/google/go-cmp/cmp/report_compare.go

vendor/github.com/google/go-cmp/cmp/report_reflect.go