PHPUnit bootstrap: Fix syntax error in error message output

[mukeshpanchal27]

Trac ticket: https://core.trac.wordpress.org/ticket/65818

Changed unconventional comma operator to semicolon for better readability and conventional PHP style.

- echo 'Please make sure they are installed and enabled.' . PHP_EOL,
+ echo 'Please make sure they are installed and enabled.' . PHP_EOL;

While the comma operator is technically valid PHP syntax, using a semicolon is the conventional and more readable approach.

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[SergeyBiryukov]

Thanks for the PR! Merged in r63300.