Remove GLOB_BRACE from unittest functions.

[amykamala]

[ from @sawanoboly ​https://github.com/WordPress/phpunit-test-runner/issues/128 ]

"One of the unit tests uses the GLOB_BRACE option for glob.

​https://github.com/digitalcube/wordpress-develop/blob/master/tests/phpunit/tests/image/functions.php#L22-L27

The GLOB_BRACE option is not supported by some libc-compatible implementations. Solaris is shown as an example in the documentation, but it cannot use this option on other implementations, such as alpine linux, which uses musl-libc.

​https://www.php.net/manual/en/function.glob.php

I'd like to see them switch to an implementation that doesn't use GLOB_BRACE, as it seems to be just cleaning up the files, regardless of compatibility."

[SergeyBiryukov]

In 49450:

Tests: Remove the GLOB_BRACE flag for glob() function from unit tests.

The flag was previously used in PDF preview tests when cleaning up temporary files.

As noted in the PHP documentation, the flag is not available on some non-GNU systems, like Solaris or Alpine Linux.

Follow-up to [40130].

Props amykamala, sawanoboly.
Fixes #51661.