fix: guard each screenshot directory the way the parent is guarded

[yeapea]

Screenshot::cleanup() guards the parent directory with is_dir() and then calls rmdir() on
three children without one. Two of those — Sliders and ImageDiffView — belong to the
visual-diff features, so an ordinary run never creates them.

The @ silences the message, but PHPUnit still records a suppressed diagnostic as a
Test-Runner-Triggered PHP Warning. A suite running with failOnWarning="true" therefore exits
non-zero while every test passes:

Test Runner Triggered 2 PHP Warnings
  src/Support/Screenshot.php:83  rmdir(...): No such file or directory
8 passed, exit code 1

Why the existing guard does not catch it

glob() on a missing directory answers with an empty array, not false. So is_array($files)
is true, the unlink loop runs over nothing, and control reaches the rmdir() that warns.

Why it is easy to miss

cleanup() early-returns while the screenshots directory does not exist. So the first run in a
fresh tree is green and only the second one warns — the first run creates the directory that
makes the next one warn. CI that checks out a clean workspace per run never sees it; running the
browser suite twice in a row locally reproduces it every time.

The change

Each directory is guarded the way the parent already is. It is additive and cannot alter behaviour
for a directory that exists — the loop body is unchanged for those.

Verified, both arms

On PHP 8.5 with failOnWarning="true", running the browser suite twice in a row:

	first run	second run
before	exit 0, 8 passed	exit 1, 2 runner-triggered warnings, 8 passed
after	exit 0, 8 passed	exit 0, 0 warnings, 8 passed

Note

#229 also touches this file (configurable screenshot directory). The two do not overlap in intent —
this one only adds the missing guard inside the existing loop — but whichever lands second will want
a trivial rebase.