`py-kill-buffer-unconditional', new command

 Kill buffer unconditional; if buffer-process exists, kill it first. 
 Sometimes being queried sucks...

 Used with tests here
`py-execute-prepare' fixed

Provide default shell if not given

in devel:
write-unified-extended-execute-forms added
ert tests prefix normalized `py-ert-'

new `py-ert-execute-statement-test'
Provide `py-kill-buffer-unconditional', used by tests
test/python-mode-ert-tests.el renamed test/py-ert-tests.el

makes tests easier by not creating a another recent buffer having
"python" in name
`py-postprocess-output-buffer' fixed

Error-message should point to orig-file/line, not to the temporary
file executed in reality

tests fixed