Support Summer '18 option to Skip Code Coverage

[Jason Clark]

Per the release notes, "Opt Out of Collecting Code Coverage Information During Test Runs":

Speed up Apex test runs by opting out of collecting code coverage information when you want faster feedback on pass or fail status rather than coverage data. When you opt out, no data about Apex test coverage is stored.

Also per the release notes, "Tooling API New and Changed Objects", looks like ApexTestQueueItem has a new field, ShouldSkipCodeCoverage.

[Scott Wells]

Duplicate of #1002.

[Scott Wells]

Folding this into the Summer '18 support story. I've already implemented this feature and it will be included in Summer '18 pre-release builds and, once Summer '18 is available in all orgs, all official builds.