Themes: Add test for theme atttibute in file-based block template.

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

tests/phpunit/data/templates/template-with-nested-template-part.html

@@ -0,0 +1,3 @@
+<!-- wp:group -->
+<!-- wp:template-part {"slug":"header","align":"full", "tagName":"header","className":"site-header"} /-->
+<!-- /wp:group -->

tests/phpunit/data/templates/template-with-template-part-with-existing-theme-attribute.html

@@ -0,0 +1 @@
+<!-- wp:template-part {"slug":"header","theme":"fake-theme","align":"full", "tagName":"header","className":"site-header"} /-->

tests/phpunit/data/templates/template-with-template-part.html

@@ -0,0 +1 @@
+<!-- wp:template-part {"slug":"header","align":"full", "tagName":"header","className":"site-header"} /-->