[test] Change tests of optional<T> to use operator bool()

This is part one of the fix for the build on osx, issue #181.
ImplContainer: Add operator!=(bool b)

We already had operator==(bool b). This is part two of the fix for
the build on osx, issue #181.
Merge pull request #184 from gicmo/osxBuild

Fix build on OSX. Closes issue #181