fix: honor screenshot pixel tolerances

[lazerg]

expectScreenshot() compares the raw PNG bytes with toMatchSnapshot() first, and only asks Playwright for a pixel comparison once those differ. But both paths out of the catch throw, so a screenshot Playwright reports as a match still fails the test, and maxDiffPixels, threshold and detectAntialiasing never get to decide anything. Chromium's PNG output is not byte-reproducible, so a handful of antialiased pixels is enough to fail a run at random.

Playwright signals a real mismatch by erroring on the expectScreenshot call, which the websocket client already turns into a failed expectation before the loop below it runs. Dropping the trailing throw therefore keeps genuine regressions failing and lets the tolerances apply.

To reproduce, take a screenshot assertion with a committed baseline and re-encode the stored snapshot so the pixels stay identical but the bytes change (appending a PNG tEXt chunk does it). The test fails today and passes with this change.

Fixes pestphp/pest#1881