Revert "test: unit/popen"

Found another problem with the test:

 | /builds/DtQXhC5e/0/tarantool/tarantool/test/unit/popen.c:63:6:
 | error: variable 'rc' is used uninitialized whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
 |        if (handle == NULL)
 |            ^~~~~~~~~~~~~~

Decided to revert it and fix in background.

This reverts commit 40a51647.