gh-159394: Use current screen size in test_newterm

[lpyu001]

• Issue: test_curses regression in 3.16.0a1: test_newterm fails on a real terminal that's not 80x25 #159394

[lpyu001]

cc @serhiy-storchaka

[serhiy-storchaka]

Thank you for the PR, @lpyu001. Unfortunately, comparing with curses.LINES and curses.COLS makes the check almost a tautology, since newterm() sets them from the same screen, and it does not fix SLKTests.test_init_reserves_a_line, which fails for the same reason. The size of the screen depends on the LINES and COLUMNS environment variables (see the comment in the issue), so #159665 sets them in these tests instead.