[TestPluginCommands] Add skipIfGcc at the right place.

Summary:
The decorator @expectedFailureGcc was at the wrong place skipping this
test all together. This patch removes it and adds skipIfGcc at the right
place.

Test Plan: dotest.py -C <gcc|clang> -p TestPluginCommands

Reviewers: chying, chaoren

Reviewed By: chaoren

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D9010

llvm-svn: 234821