Added cache clear to e2e scripts

[ro-savage]

Fixes #2399

Yarn is caching old react-scripts when running the e2e tests, which means old versions are being used during the build process. Fine for CI where each build is on a clean machine but breaks e2e in unforeseen ways on local machines.

This might be related to yarnpkg/yarn#2165

The fix is to run yarn clean cache before running the e2e tests.

[gaearon]

Looks like that command doesn't work on AppVeyor.
Probably yarnpkg/yarn#2591.

Worth trying yarnpkg/yarn#2591 (comment)?

[ro-savage]

Issue is fixed in latest version of yarn, so decide best thing to do is just install the latest version of yarn. Once AppVeyor updates their yarn, the npm install -g yarn can be removed.

AppVeyor issue: appveyor/ci/issues/1576

However, now there is a new issue related to #2030. Might be caused by using the latest version of yarn on windows machines.

Could either wait for the fix or try change this PR to use the work around gaearon mentioned.

# Workaround for https://github.com/yarnpkg/yarn/issues/2591 case "$(uname -s)" in *CYGWIN*|MSYS*|MINGW*) yarn=yarn.cmd;; *) yarn=yarn;; esac eval $yarn run build

[ro-savage]

Updated to apply the yarn=yarn.cmd fix, rather than installing the latest yarn which causes issues else where. The latest version of yarn + cra + windows 10 issue can be dealt with in #2030

[gaearon]

Is this hack still necessary? AppVeyor has been failing for a while now. I tried reverting this in #2626 but it doesn't help. I'm super confused.