Fix TestNameLookup for GCC

Summary:
GCC emits also symbols for the __PRETTY_FUNCTION__ virtual variable,
which we accidentaly pick up when looking for functions for with
"unique_function_name" in the name. This makes the target.FindFunctions
call fail, as that symbol is not a function.

I also strenghten the test a bit to make sure we actually find all the
functions we are interested in. I've put a check that we find at least 6
functions, but maybe this should be *exactly* 6 ?

Reviewers: clayborg

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D29932

llvm-svn: 295170