[lldb/test] Fix failure caused by synthetic symbol name refactoring

This patch fixes a failure in `TestFunctionStarts.py` that appeared
following a change of implementation for synthetic symbol names:

https://reviews.llvm.org/D105160

The failure is caused because the previously mentioned patch removes the
object file basename from the generated synthetic symbol names to allow
them to be shared in the constant string pool.

Hence, that last check is not necessary anymore.

rdar://80092322

Differential Revision: https://reviews.llvm.org/D105366



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>