tests: ensure that most python scripts have usage text

tests: ensure that most python scripts have usage text

When a script is run with the wrong arguments, it should at least say
something like this:

    Usage: samba-foo [OPTIONS]

For many samba scripts, especially without a server environment, having
no arguments is the wrong arguments.

Here we look for every executable file with '#![...]python[3]' on the
first line, and exclude certain files and directories that have excuses
to fail the test. For example, many selftest scripts are stream-oriented
and will hang forever waiting for stdin, which is not an error. Some
test modules are designed so they can be optionally run from the command
line, but this option is typically only used by the developer who is
writing them.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>