Add tests for len, is_empty, contains methods of cc_std::std::string/string_view

[copybara-service]

Add tests for len, is_empty, contains methods of cc_std::std::string/string_view

• Implements these methods where not already present.
• The methods are already available for string (through Deref<Target = [u8]>).
• Updates string test setup to match string_view (removes cc_std_ from folder name, adds use statement for cc_std::std::string inside test file).

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.