E2E tests: screenshot threshold set to 0.2 by default

[scurker]

This came up in a recent E2E test where the colors of an existing screenshot comparison "passed" even though the colors had changed. This appears to be due to a default threshold value set via playwright:

https://playwright.dev/docs/api/class-pageassertions#page-assertions-to-have-screenshot-1

An acceptable perceived color difference in the YIQ color space between the same pixel in compared images, between zero (strict) and one (lax), default is configurable with TestConfig.expect. Defaults to 0.2.

Since we're doing component snapshots, we want screenshots to be an exact match so we can adequately determine when a component has changed irrespective of the threshold.

We will need to update the default config to set the threshold values: https://playwright.dev/docs/api/class-testconfig#test-config-expect