The gdb.python/py-inferior-leak.exp test makes use of the tracemalloc
module. When running the Python tests with a GDB built against Python
2 I ran into a test failure due to the tracemalloc module not being
available.
This commit adds a new helper function to lib/gdb-python.exp that
checks if a named module is available. Using this we can then skip
the py-inferior-leak.exp test when the tracemalloc module is not
available.
