ZODB Components: Revert 'Allow to execute runUnitTest for bt5 Test Components' (a771dca4) to fix tests bootstrap.

The new syntax to load ZODB Tests Components is:
runUnitTest BT_TITLE:TEST_NAME

That commit was too adhoc as it was relying upon filesystem to load Tests
Components and was not behaving like any other Components (versions was not
available and other Components were not importable).

At the end, it would have meant that a Test Component ran through runUnitTest
and Live Tests (in ERP5 itself) would have behaved differently, thus instead:

1/ Install BT_TITLE dependencies and its test dependencies (new bt property to
   specify bt to be installed only for tests on a fresh instance).
2/ The site is loaded.
3/ Load the test by importing it like any other Components.