Start refactoring lldbtest.TestBase so that it inherits from a newly created lldbtest.Base class,

while its API clients remain unchanged.  The new lldbtest.Base class is to capture common behaviors
when working with the test driver to accomplish things.  The clients of lldbtest.Base can be
lldb command line and api tests as well as other generic tests like a benchmark test.

llvm-svn: 136636