Test setup helper added

[Aniket-Engg]

Fixes #1205

I have added the helper named as chai.js.
Used it in expectEvent and shouldFail helper.
Also tests who require expectEvent and shouldFail helper do not need to require chai modules again. I tested them in Ownable and PaymentSplitter tests.
If chai helper lives upto the expectation, I can work further on removing extra requires from other files of testing.

[nventuro]

Hello again @Aniket-Engg, thanks for working on this!

I wouldn't call that helper chai, that might be confuse our users into thinking we're hosting a local version of it. Maybe setup is enough?

Also, keep in mind that some tests need not only the should object to be exported, but also the BigNumber one (to create new instances, e.g. new BigNumber(3)), so we need to provide it (const { should, BigNumber } = require('setup')).

And finally, while it is true that we may be importing the same file multiple times (when also importing e.g. expectEvent), I still wouldn't remove the redundant imports, since it will be annoying down the line to figure out why chai isn't working if we ever move stuff around, remove the import from expectEvent, etc.: keeping everything explicit will reduce future pain :)

[Aniket-Engg]

@nventuro I assume i got your words right. Have made changes accordingly, please review.

[nventuro]

Hey @Aniket-Engg, thanks for following up with this! Left a couple comments clarifying what I meant in my previous comment, but this looks almost ready :)

[Aniket-Engg]

@nventuro I think now this is ready to be implemented in other files. What you think?

[nventuro]

Looks great, I think we can add this everywhere now!

[nventuro]

Looks like the CI job wasn't triggered :/ This is rather weird, I'll look into it.

[nventuro]

Seems like it was related to the PR not being mergeable - after creating a merge commit it is now working again.

[nventuro]

Awesome, thanks a lot @Aniket-Engg! At first I thought a couple BigNumber exports were missing, but that was just because it was needed for chai-bignumber.

[Aniket-Engg]

Yeah, i checked each file deeply. 😉