x/pkgsite: inconsistent text render output in screentest

[jamalc]

The testcase for "test empty jump to modal" in tests/screentest/testcases.txt occasionally fails because the font rendering on the Close button is inconsistent. Here is a similar issue with puppeteer, it seems to be limited to linux platforms in both cases. There might be some combination of chrome flags we can add to the headless-shell command to disable font smoothing or antialiasing.

[gopherbot]

Change https://golang.org/cl/383198 mentions this issue: test/screentest: disable flaky test