Reduce strain on CI

This moves 8 tasks from the PR CI configuration to the nightly and
moves most partest tasks from the nightly to a new weekly build.

N.B.: We should not be doing this, but given the load on the CI we
probably have no choice. The full tests should be introduced ASAP.
Merge pull request #1238 from gzm0/reduce-ci-strain

Reduce strain on CI
Remove failing bootstrapping test.

The tools do not compile on JDK6 with Scala 2.11.x. (see #1235)
Merge pull request #1237 from gzm0/com-js-env

Add ComJSEnv
Merge pull request #1243 from gzm0/remove-jdk6-2.11-tools

Remove failing bootstrapping test.
Flatten Semantics (remove CheckedBehaviors)
Cleanup dependency data structures
Get rid of unnamed tuple in CompleteClasspath
Fix #1244: Track and verify required semantic compliance
Fix #1240: Add stop() to AsyncJSEnv
Merge pull request #1245 from gzm0/required-semantics

Track and verify required semantics
Merge pull request #1246 from gzm0/async-stop

Fix #1240: Add stop() to AsyncJSEnv
Introduce Scala.js specific checkfile overrides
Configure semantics individually for partests
Add --showDiff flag to partest
Merge pull request #1248 from gzm0/improve-partest

Improve partest
rename to test interface
sbt compatible test iface definition