[internal] Replace var assignments with let in ui tests

[epixa]

This change was applied to any .js files under __tests__ directories in
the ui module.

This was an automatic replacement from var to let for any variable
declaration that doubles as the initial assignment. Ultimately we want
most of these to be converted to const, but this commit is so large that
it warrants breaking each step of automation up into its own commit.

For example:

var foo = 'bar'; becomes let foo = 'var';

This was accomplished by replacing:
find: var ([a-zA-Z_$][0-9a-zA-Z_$]*)(\s+)=
replace: let $1$2=

[epixa]

This is just a little too big to show in a single diff on github, but it's only touching __tests__ files, and you can see from the build whether those tests are running successfully. I'll break it up into separate PRs if requested, though.

[spalger]

I am using the following command to view the entire change set locally

# install diff-so-fancy if you haven't already
brew install diff-so-fancy

git show HEAD --patch --color | diff-so-fancy | less -R

[epixa]

You're so fancy.

[spalger]

LGTM

[panda01]

Ah i just read the description a bit more... LGTM!