Update pm-enable tests and fix underlying issues with --resolve-dependencies

[jonhattan]

Updated pm-enable tests:

1. Test cache was cleared/rebuild for D8 also
2. Don't skip the full test! just avoid doing concrete tests
3. Test pm-enable's resolve-dependencies for D6,7,8 (it was doing it only for D7). Expected to fail in D8 because of Infinite loop when trying to enable not available extensions in Drupal 8 #5
4. Add a new test to resolve-dependencies with projects namespace. Expected to fail for D7 because of Support for project namespaces #1731

Fixes

• Support for project namespaces #1731
• rln command. Due to changes in d.o html.
• Infinite loop when trying to enable not available extensions in Drupal 8 #5