Enable the libc++ tests on linux

Summary:
The list of loaded modules which skip_if_library_missing is depending on is not available on
linux until after we run the target. This causes the tests to be wrongfully skipped. This commit
moves the skip call after the run command.

Reviewers: granata.enrico, tfiala

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D13985

llvm-svn: 251102