[InlineTest] getRerunArgs returns an empty string if the test was skipped.

Summary:
The main issue that this patch is trying to address is that the current
implementation of getRerunArgs of InlineTest relies on the attribute
'using_dsym' which could be absent if the test was skipped altogether.
[That is, if both dsym and dwarf tests were skipped.]

While at it, the use of deprecated Python module 'new' is eliminated.

Test Plan: [Linux] dotest.py -p TestExprPathSynthetic

Reviewers: vharron, clayborg

Reviewed By: clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D6888

llvm-svn: 225496