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/variant_string.go

@@ -0,0 +1,27 @@
+// Code generated by "stringer -type=Variant -trimprefix=Variant -linecomment"; DO NOT EDIT.
+
+package guid
+
+import "strconv"
+
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[VariantUnknown-0]
+	_ = x[VariantNCS-1]
+	_ = x[VariantRFC4122-2]
+	_ = x[VariantMicrosoft-3]
+	_ = x[VariantFuture-4]
+}
+
+const _Variant_name = "UnknownNCSRFC 4122MicrosoftFuture"
+
+var _Variant_index = [...]uint8{0, 7, 10, 18, 27, 33}
+
+func (i Variant) String() string {
+	if i >= Variant(len(_Variant_index)-1) {
+		return "Variant(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _Variant_name[_Variant_index[i]:_Variant_index[i+1]]
+}