Add mocha options to solcoverjs

[cgewecke]

Lets you write tests with a grep-able tag...

it("is a gas usage simulation [ @fails-coverage ]", async function(){
...
});

...and skip them via solcoverjs

module.exports = {
  mocha: {
    grep: "@fails-coverage",
    invert: true
  }
}

mocha options link
(PR also has some additional tests cleanup).