cli/command/context: test inspecting context with custom metadata

The CLI does not currently expose options to add custom metadata to
contexts, but contexts support them.

- update test-utilities to allow setting custom metadata
- update the inspect test to verify that custom metadata is included
  when inspecting a context.
- update the import/export tests to verify that custom metadata
  is preserved.

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

cli/command/context/update_test.go

@@ -34,7 +34,7 @@ func TestUpdateDescriptionOnly(t *testing.T) {
 
 func TestUpdateDockerOnly(t *testing.T) {
 	cli := makeFakeCli(t)
-	createTestContext(t, cli, "test")
+	createTestContext(t, cli, "test", nil)
 	assert.NilError(t, RunUpdate(cli, &UpdateOptions{
 		Name: "test",
 		Docker: map[string]string{