[Test Suite] truffle test on all ignored by coverage

[bh2smith]

Some gas estimation tests are ignored by solidity-coverage in CI via the @skip-on-coverage tag (using mocha -g). However, it is important that these tests are run via truffle test. At the moment, we have to put the test in a separate file and run truffle test on this separate file.

Truffle currently has an open issue trufflesuite/truffle#2080 that would help ensure that no tests are accidentally untested via either solidity-coverage or truffle test.

This issue was open because of the discussion in #486.