* tests/macros.h (CONTINUE_AFTER_ASSERT): New macro.
(test_exit_status): New variable.
(ASSERT, ASSERT_NO_STDIO): If CONTINUE_AFTER_ASSERT is 1, set
test_exit_status instead of aborting.
* tests/**/test-*.{c,h,cc} (main): Instead of exiting with exit code 0,
exit with exit code test_exit_status.
* tests/test-spawn-pipe-child.c: Undefine CONTINUE_AFTER_ASSERT.
* tests/uninorm/test-nf*.c: Include macros.h.
* tests/uninorm/test-u32-nf*-big.c: Likewise.
* tests/random*.c: Define NO_MAIN_HERE.
* tests/test-vsprintf-gnu.c: New file, based on
tests/test-vsprintf-posix.c.
* tests/test-sprintf-gnu.h: New file, based on
tests/test-vasnprintf-gnu.c.
* modules/vsprintf-gnu-tests: New file, based on
modules/vsprintf-posix-tests.
