GUID tests to use t.Run (#256)

Switched to subtests using `t.Run()` for GUID testing, rather than using
log statements to demarcate different tests.

Added `.String` to GUID `Variant` and `Version` using
`golang.org/x/tools/cmd/stringer`.

Added `tools.go` to version `stringer` in go.mod and allow
`go generate ./...` to be run without needing a `go get` call.
Based off of [go wiki](https://github.com/golang/go/wiki/Modules#how-can-i-track-tool-dependencies-for-a-module)
recommendation.

Signed-off-by: Hamza El-Saawy <hamzaelsaawy@microsoft.com>

Signed-off-by: Hamza El-Saawy <hamzaelsaawy@microsoft.com>

pkg/guid/guid.go

@@ -14,17 +14,19 @@ import (
 	"strconv"
 )
 
+//go:generate go run golang.org/x/tools/cmd/stringer -type=Variant -trimprefix=Variant -linecomment
+
 // Variant specifies which GUID variant (or "type") of the GUID. It determines
 // how the entirety of the rest of the GUID is interpreted.
 type Variant uint8
 
-// The variants specified by RFC 4122.
+// The variants specified by RFC 4122 section 4.1.1.
 const (
 	// VariantUnknown specifies a GUID variant which does not conform to one of
 	// the variant encodings specified in RFC 4122.
 	VariantUnknown Variant = iota
 	VariantNCS
-	VariantRFC4122
+	VariantRFC4122 // RFC 4122
 	VariantMicrosoft
 	VariantFuture
 )
@@ -34,6 +36,10 @@ const (
 // hash of an input string.
 type Version uint8
 
+func (v Version) String() string {
+	return strconv.FormatUint(uint64(v), 10)
+}
+
 var _ = (encoding.TextMarshaler)(GUID{})
 var _ = (encoding.TextUnmarshaler)(&GUID{})