add group tag on tests covering ASTTraitMethodCollisionException
add a new test playing the issue 154

No ASTTraitMethodCollisionException is thrown if only one method is not abstract.
allow multiple identical methods from trait, if they are abstract.
Merge branch 'issue-154' of https://github.com/Raphhh/pdepend into Raphhh-issue-154
Closes GH154: Keep current behavior but keet least abstract method
Merge branch 'Raphhh-issue-154'