avoid stack corruption due to large arrays

valgrind complains (and it is probably right) that stack is being
corrupted in deadlock5 and deadlock6. It is due to large arrays being
allocated on stack. This is solved by allocating them in static memory
(BSS region).

Valgrind & drobne testy

Flush stdout in extended_hello test

fix bug in ASSERT_ZERO

`errno` global variable was queried for error code instead of checking
content of `_errno` local variable.

Kilka testów

Test enhancements

Drobna poprawka do testów

for better debug experience.

This is UB.

It can be used for checking equality with any MIMPI_Retcode.
In a sense, it extends `ASSERT_MIMPI_OK`.

This is a good practice in header files to prevent duplicated
definition errors when linking.

pipe_closed: allow `MIMPI_SUCCESS` on send to finishing process

Kilka prostych testów

Add tests for premature finish of one process