integration-cli: remove isCpCannotCopyReadOnly utility

This utility was just string-matching error output, and no longer had a
direct connection with ErrContainerRootfsReadonly / ErrVolumeReadonly.
Moving it inline better shows what it's actually checking for.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

integration-cli/docker_cli_cp_to_container_test.go

@@ -411,9 +411,7 @@ func (s *DockerCLICpSuite) TestCpToErrReadOnlyRootfs(c *testing.T) {
 	dstPath := containerCpPath(containerID, "/root/shouldNotExist")
 
 	err := runDockerCp(c, srcPath, dstPath)
-	assert.ErrorContains(c, err, "")
-
-	assert.Assert(c, isCpCannotCopyReadOnly(err), "expected ErrContainerRootfsReadonly error, but got %T: %s", err, err)
+	assert.ErrorContains(c, err, "marked read-only")
 	assert.NilError(c, containerStartOutputEquals(c, containerID, ""), "dstPath should not have existed")
 }
 
@@ -436,8 +434,7 @@ func (s *DockerCLICpSuite) TestCpToErrReadOnlyVolume(c *testing.T) {
 	dstPath := containerCpPath(containerID, "/vol_ro/shouldNotExist")
 
 	err := runDockerCp(c, srcPath, dstPath)
-	assert.ErrorContains(c, err, "")
+	assert.ErrorContains(c, err, "marked read-only")
 
-	assert.Assert(c, isCpCannotCopyReadOnly(err), "expected ErrVolumeReadonly error, but got %T: %s", err, err)
 	assert.NilError(c, containerStartOutputEquals(c, containerID, ""), "dstPath should not have existed")
 }

integration-cli/docker_cli_cp_utils_test.go

@@ -232,10 +232,6 @@ func isCpCannotCopyDir(err error) bool {
 	return strings.Contains(err.Error(), archive.ErrCannotCopyDir.Error())
 }
 
-func isCpCannotCopyReadOnly(err error) bool {
-	return strings.Contains(err.Error(), "marked read-only")
-}
-
 func fileContentEquals(c *testing.T, filename, contents string) error {
 	c.Helper()