Fixes for logic related to FEATURES=test to USE=test mapping:

Fixes for logic related to FEATURES=test to USE=test mapping:
* Add EBUILD_FORCE_TEST to the environment whitelist and filter
  it from the saved environment in save_ebuild_env().
* Tweak logic inside the ebuild command and config.regenerate()
  so that EBUILD_FORCE_TEST works even in odd cases like when
  USE=test is masked.
* Only make FEATURES=test map to USE=test when "test" is
  actually in IUSE.
* Remove USE=test from the set of implicit IUSE so that useq()
  calls in ebuild.sh properly generate a QA Notice when "test"
  is missing from IUSE.

svn path=/main/trunk/; revision=9064