net/url: add test of "Windows" file URL

This is just testing the status quo, so that any future attempt
to change it will make the test break and redirect the person
making the change to look at issue 6027.

Fixes #6027.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/83930046

src/pkg/net/url/url_test.go

@@ -251,6 +251,17 @@ var urltests = []URLTest{
 		},
 		"file:///home/adg/rabbits",
 	},
+	// "Windows" paths are no exception to the rule.
+	// See golang.org/issue/6027, especially comment #9.
+	{
+		"file:///C:/FooBar/Baz.txt",
+		&URL{
+			Scheme: "file",
+			Host:   "",
+			Path:   "/C:/FooBar/Baz.txt",
+		},
+		"file:///C:/FooBar/Baz.txt",
+	},
 	// case-insensitive scheme
 	{
 		"MaIlTo:webmaster@golang.org",