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/config.go

@@ -14,6 +14,7 @@ type Package struct {
 	Version    string     `json:"version"`     // git sha, tag or branch
 	Generate   [][]string `json:"generate"`    // generate commands to run
 	Dir        string     `json:"dir"`         // working directory for generate commands
+	Test       string     `json:"test"`        // test path relative to repo root (if empty defaults to ./...)
 }
 
 // Name returns the package name.
@@ -26,6 +27,14 @@ func (p Package) CloneURL() string {
 	return "https://" + p.ImportPath + ".git"
 }
 
+// TestPath returns the paths to run "go test" on, relative to the repository root.
+func (p Package) TestPath() string {
+	if p.Test == "" {
+		return "./..."
+	}
+	return p.Test
+}
+
 // LoadPackages loads a list of package configurations from JSON format.
 func LoadPackages(r io.Reader) ([]Package, error) {
 	var pkgs []Package