cmd/testwrapper: apply results of all unit tests to coverage for all packages

This allows coverage from tests that hit multiple packages at once
to be reflected in all those packages' coverage.

Updates #cleanup

Signed-off-by: Percy Wegmann <percy@tailscale.com>

cmd/testwrapper/testwrapper.go

@@ -278,6 +278,7 @@ type nextRun struct {
 				goTestArgsWithCoverage,
 				fmt.Sprintf("-coverprofile=%v", coverageFile),
 				"-covermode=set",
+				"-coverpkg=./...",
 			)
 		}