Add tests for php exist pattern in SAST template

Adam Cohenrequested to merge
add-tests-for-php-in-sast-template into master

What does this MR do and why?

Adds the tests for php exist patterns in the SAST template, originally implemented in Adjust SAST exists rule for php (!200262 - merged) • Adam Cohen • 18.4.

References

Refactor php-related SAST rules (#559997 - closed) • Adam Cohen • 18.4

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