Performs minor code improvements in abstract-testcase.php

[andizer]

I've done some minor code improvements to make the code in abstract-testcase a little bit better:

• Added TRUE as third argument for in_array checks
• Used strpos instead of substr where possible
• Add value directly to array by key instead of array_push
• Removed an else statement because the if already returns a value.

I hope these changes are desirable. I would love to hear this, because I can continue performing equal changes elsewhere in the testing framework.

[SergeyBiryukov]

In 44916:

Coding Standards: Minor code and performance improvements in phpunit/includes/abstract-testcase.php.

Props andizer.
Fixes #46500.