tests/thirdparty: add klauspost/compress/s2 (#131)

Adds @klauspost's S2 implementation to the third party test suite.

The full klauspost/compress tests are slow but we only care about the S2 sub-package. Therefore this PR also adds the option to only run a subset of the package tests, controlled by a "test" parameter in the JSON configuration.

Closes #130

tests/thirdparty/packages_test.go

@@ -145,7 +145,7 @@ func (t *PackageTest) diff() {
 
 // test runs go test.
 func (t *PackageTest) test() {
-	t.gotool("test", "./...")
+	t.gotool("test", t.TestPath())
 }
 
 // git runs a git command.