Publish testing suite and behavior fixtures

[nventuro]

This will probably be the biggest 2.1 feature: we want to publish our testing suite on the npm package, along with helpers, behaviors, etc., so users can more easily and correctly test their code. A lot of work is required before this can be achieved though:

• Add a testing guide (Add testing guide #1077)
• Improve test descriptions (Improve test suite descriptions #1157)
• Test the helpers (Test the testing helpers #1078)
• Improve the EVM exception helpers (unify ethereum revert/throw/etc error expectations #942)
• Improve the logging helpers (Add BigNumber support to expectEvent/inLogs #1026)
• Set standards for our behaviors and document them (Properly document and maintain contract behavior test files #1085)

[ross-rosario]

I wonder if we should extract tests/helpers directory into its own package, as a step 0 in this milestone. It will be useful immediately, until all remaining issues above are done.

I'm happy to create PR if @nventuro sees that fit or do anything else to help with this endeavor.

[nventuro]

https://github.com/OpenZeppelin/openzeppelin-test-helpers is now a thing!