cmd/go: do not cover package unsafe

Even if requested, there is no .go file for unsafe - it comes from the
compiler - so referencing its cover variables will break the compilation
in a command like
	go test -coverpkg=all fmt

Fixes #10408.

Change-Id: If92658ef6c29dc020f66ba30b02eaa796f7205e0
Reviewed-on: https://go-review.googlesource.com/8891
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/test.go

@@ -430,6 +430,10 @@ func runTest(cmd *Command, args []string) {
 
 		// Mark all the coverage packages for rebuilding with coverage.
 		for _, p := range testCoverPkgs {
+			// There is nothing to cover in package unsafe; it comes from the compiler.
+			if p.ImportPath == "unsafe" {
+				continue
+			}
 			p.Stale = true // rebuild
 			p.fake = true  // do not warn about rebuild
 			p.coverMode = testCoverMode