🏗 Print logs from chromedriver when visual diff tests fail

[rsimha]

We're trying to get to the bottom of some selenium errors while running the visual tests. They've been elusive while debugging, but appear frequently in the wild.

This PR prints the last 100 lines from chromedriver.log when failures do occur. It also cleans up an earlier fix that didn't work.

To aid in the debugging of #13700

[rsimha]

/to @erwinmombay @choumx
/cc @timhaines

[timhaines]

@rsimha Nice - please do feel welcome to email me or @me links if/when you see problems related to this.

[rsimha]

Here's what the last 100 lines of chromedriver.log look like during a successful build: https://gist.github.com/rsimha/e4239983f7c3810da2a7eff69eba488e

[rsimha]

Tested this locally to make sure the error message and logs get printed. This is now ready to merge.

pr-check.js: Done running gulp visual-diff --headless Total time: 1m 47s
pr-check.js: ERROR: Found errors while running gulp visual-diff --headless. Here are the last 100 lines from chromedriver.log:

<logs>