[gdb/testsuite] Reimplement gdb.gdb/python-selftest.exp as unittest

[gdb/testsuite] Reimplement gdb.gdb/python-selftest.exp as unittest

The test-case gdb.gdb/python-selftest.exp:
- patches the gdb_python_initialized variable in gdb to 0
- checks that the output of a python command is "Python not initialized"

Reimplement gdb.gdb/python-selftest.exp as unittest, using:
- execute_command_to_string to capture the output
- try/catch to catch the "Python not initialized" exception.

Tested on x86_64-linux.