While we already have unit test coverage for `_inject_theme_attribute_in_block_template_content`, those tests only verify that ''that'' function does what is supposed to do; there's however no guarantee that `_build_block_template_result_from_file` uses that function (or whatever other technique) to actually inject the theme attribute.
This patch adds test coverage to verify that the theme attribute is correctly injected by `_build_block_template_result_from_file`.
Props costdev, gziolo, mukesh27, spacedmonkey.
Fixes#59325. See #59313.
git-svn-id: https://develop.svn.wordpress.org/trunk@56562 602fd350-edb4-49c9-b593-d223f7449a82
