tests: migrate strings.Contains -> is.Contains in assertions

Migrated using

  find . -type f -name "*_test.go" |
    xargs gofmt -w \
      -r "assert.Check(t, strings.Contains(a, b)) -> assert.Check(t, is.Contains(a, b))"

  find . -type f -name "*_test.go" |
    xargs gofmt -w \
      -r "assert.Assert(t, strings.Contains(a, b)) -> assert.Assert(t, is.Contains(a, b))"

Using a boolean in assert.Assert or assert.Check results in error
messages that don't contain the actual problematic string, and when
running the integration suite on an actual machine (where the source
code parsing doesn't work) this makes it almost impossible to figure out
what the actual error is.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

integration-cli/docker_api_attach_test.go

@@ -96,7 +96,7 @@ func (s *DockerAPISuite) TestGetContainersWsAttachContainerNotFound(c *testing.T
 	b, err := request.ReadBody(body)
 	assert.NilError(c, err)
 	expected := "No such container: doesnotexist"
-	assert.Assert(c, strings.Contains(getErrorMessage(c, b), expected))
+	assert.Assert(c, is.Contains(getErrorMessage(c, b), expected))
 }
 
 func (s *DockerAPISuite) TestPostContainersAttach(c *testing.T) {