Incorrect check for block theme in legacy PHPUnit test

[flixos90]

The very old test_switch_theme() recently required updates, and at the same time was updated to not cover block themes, as the test is overly complex and not up to date with how PHPUnit tests should be written as of today. Still, simply removing is probably not a good idea. Ideally it could be refactored, but that's also a very low hanging fruit.

In any case, it was recently noticed that the test unintentionally was still covering block themes in certain conditions, due to a bug in the condition. Here's the problem and solution:

• The block-templates theme support for block themes is added dynamically via wp_enable_block_templates(), so checking for that isn't reliable.
• The conditions used in wp_enable_block_templates() should be used instead.

The check for whether a theme shouldn't be covered in the old test_switch_theme() method may be inaccurate because:

• The block-templates theme support for block themes is added dynamically via wp_enable_block_templates(), so checking for that isn't reliable.
• The conditions used in wp_enable_block_templates() should be used instead.

Related to #5634.

Trac ticket: https://core.trac.wordpress.org/ticket/59881

[flixos90]

In 57104:

Tests: Fix incorrect check for block themes in legacy PHPUnit test for themes.

Props mukesh27.
Fixes #59881.

Committed in https://core.trac.wordpress.org/changeset/57104