Blocks: Ensure string return value from render_callback

[aduth]

See: #4716 (comment)

This pull request seeks to update and verify the behavior of WP_Block_Type to ensure a string return value from its render function, as documented in inline code docs.

Testing instructions:

Verify PHPUnit tests pass:

phpunit

cc @tfrommen

[jorgefilipecosta]

Our CI tests were failing but I restarted the job and now they are passing. The changes look good to me 👍

[johnbillion]

The test_dynamic_block_renders_string() test is useless as it will pass without the (string) cast in place. assertSame() needs to be used in place of assertEquals() to assert that the type is the same, or, preferably, assertInternalType( 'string', ... ).

[aduth]

Oof, good catch @johnbillion, I wrongly expected assertEquals to be a strict equality check. I'll open a follow-up pull request.

[johnbillion]

Yeah it's an annoying one, that.