Custom Logo: Add unit tests

[obenland]

[36698] introduced custom logos. Let's add some unit tests to make sure the API doesn't break with future changes.

See #33755.

[obenland]

@jorbin, would you mind reviewing these when you get a chance?

[jorbin]

test_the_custom_logo feels like it's just making sure that the_custom_logo is a wrapper to echo get_custom_logo. Would it make more sense to test expected HTML output?

Calling _remove_custom_logo() after an assertion means that it may not be run if the assertion fails. We should add that to the teardown.

Tests_WP_Custom_Logo doesn't use the wp_custom_logo property. Looks like it can be eliminated.

[obenland]

Thanks @jorbin, I updated the patch!

[jorbin]

In 36905:

Add Automated Tests for custom logo support

[36698] introduced custom logos, this tests the API.

Fixes #36086. See #33755.
Props obenland.

[ocean90]

The output in test_the_custom_logo is different when Twenty Sixteen is active, see ​https://travis-ci.org/aaronjorbin/develop.wordpress/jobs/114894614#L1183.

[obenland]

In 36911:

Tests: Account for expected attachemnt size in custom logo output.

Fixes #36086.

[obenland]

In 36914:

Tests: Let's not reinvent wp_get_attachment_image().

See ​https://travis-ci.org/aaronjorbin/develop.wordpress/builds/114913862
See #36086.

[obenland]

In 36949:

Tests: Introduce multisite unit tests.

Makes sure custom logo functions work for other sites within a network.
Fixes a bug in get_custom_logo() where the correct logo was returned, but
linked to the wrong site.

H/t ocean90.
See #33755, #36086.

[johnbillion]

In 36952:

Correct a unit test for custom icon functionality.

See #36086