Report a failing Ember suite instead of exiting the process

[tricknotes]

Summary

Shell#test ran ember test through Runner#run!, which calls exit when the command fails. App#test therefore could never return the false its success? call promises: any consumer running the suite from Ruby was terminated mid-run.

The gem's own RSpec suite is such a consumer — when the EmberCli::App#test example's ember test run fails (e.g. no Chrome available), the whole RSpec process exits at that point, reporting however many examples had run so far with 0 failures. Observed as example counts varying between 2 and 103 across runs of the same revision.

Changes:

• Shell#test runs the suite with Runner#run and returns the status, so App#test reports the outcome to its caller
• EmberCli.test! raises EmberCli::TestFailureError (naming the failing application) when a suite fails, so library consumers can rescue the failure in Ruby instead of having their process terminated; the exception still exits rake ember:test nonzero
• Runner#run! keeps its exiting behavior for compile and install, whose rake tasks rely on it (its spec documents the SystemExit)
• Add specs for Shell#test (both outcomes) and EmberCli.test! (runs every suite; raises on failure), and a CHANGELOG entry

Verification

• spec/lib now runs to completion deterministically: the same example count on every run (previously the count varied per seed), with the one genuine failure on this machine being the Chrome-dependent App#test example — reported as a failure instead of aborting the suite
• On CI, where Chrome is available, that example passes as before