runtimetest: fix root readonly check

The rootfs might not be readable despite spec.Root.Readonly being false
and that's not a spec violation. Example: the rootfs belong to an
unmapped uid.

This test is useful for validation/root_readonly_true.go

Delete validation/root_readonly_false.go since that's not a spec
violation.

Signed-off-by: Alban Crequy <alban@kinvolk.io>

Author: alban

cmd/runtimetest/main.go

@@ -328,12 +328,7 @@ func validateRootFS(spec *rspec.Spec) error {
 		if err == nil {
 			return specerror.NewError(specerror.RootReadonlyImplement, fmt.Errorf("rootfs must be readonly"), rspec.Version)
 		}
-	} else {
-		err := testWriteAccess("/")
-		if err != nil {
-			return specerror.NewError(specerror.RootReadonlyImplement, fmt.Errorf("rootfs must not be readonly"), rspec.Version)
-		}
-	}
+	} // no need to check the else case: unwriteable root is not a spec violation
 
 	return nil
 }