Update corpus test expectations for MultiplyAlpha() change

Tweaks test expectations now that MultiplyAlpha() takes a float as-is,
instead of rounding. See https://pdfium-review.googlesource.com/115450

Change-Id: I58a8450e2713084dc8198d31847e6bcf9dd5ac5a
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium_tests/+/115418
Reviewed-by: Tom Sepez <tsepez@chromium.org>