[lldb] [unittests] Use non-empty format string for Timer()

Pass dummy '.' as format string for Timer() rather than an empty string,
in order to silence gcc warnings about empty format string
(-Wformat-zero-length).  The actual format string is irrelevant
to the test in question.

Differential Revision: https://reviews.llvm.org/D58680

llvm-svn: 354922