* tests/basic.c (test_basic_status, test_basic_args,
test_basic_pipeline, test_basic_wait_all, test_basic_setenv,
test_basic_unsetenv, test_basic_clearenv, test_basic_chdir,
test_basic_fchdir, test_basic_pre_exec, test_basic_sequence): Use
(void *) rather than NULL as a sentinel for variadic functions.
* tests/common.c (temp_dir_teardown): Likewise.
* tests/exec.c (test_exec_process): Likewise.
* tests/inspect.c (test_inspect_command, test_inspect_pipeline):
Likewise.
* tests/pump.c (fail_unless_files_equal,
test_pump_connect_attaches_correctly, test_pump_tee): Likewise.
* tests/redirect.c (test_redirect_files, test_redirect_outfile):
Likewise.