* python3: check ksymname calls with _assert_is_bytes

Fixes a bytes/string concatenation error when get/fix_syscall_fnname is
called from a python3 system.

* python3: use env python invocation in tools

In order to facilitate testing, but not necessarily as an example of
good practice, I am changing the invocation of the test tools to use
`/usr/bin/env python`, so that we can control which python (2 vs 3)
gets invoked for the test. On the buildbots, I plan to add an optional
`ln -s /usr/bin/python3 /usr/local/bin/python` on systems that have
python3-bcc package built. This way, we get more test coverage. Having a
cmake mechanism to enable both python2 and python3 testing could be a
further enhancement.

* tools/memleak: add an explicit stdout.flush to print loop

The stdout flush behavior seems to have changed in python3, breaking one
of the tests. I think it makes sense to flush stdout at the end of each
timed interval loop anyway, so adding that to the tool itself.

* tests: add b'' strings and fix dangling handles

Add b'' strings in a few places in the test tools, and fix one dangling
process handle in the memleak test tool runner.