[gdb/testsuite] Avoid generating gdb_leak_detector.cpython-<n>.pyc

After running test-case gdb.python/py-color-leak.exp in a container where I
don't have PYTHONDONTWRITEBYTECODE set, I get:
...
$ ls src/gdb/testsuite/gdb.python/__pycache__/
gdb_leak_detector.cpython-313.pyc
...

Fix this by setting sys.dont_write_bytecode to True in the python scripts
importing the module.

Tested on x86_64-linux.