tests: avoid compiler warnings

tests: avoid compiler warnings

Silence more warnings during coreutils build.

* tests/test-fcntl.c (main): Delete unused parameters.
* tests/test-freopen-safer.c (main): Likewise.
* tests/test-xalloc-die.c (main): Mark unused parameters.
* tests/test-fseeko.c (main): Likewise.
* tests/test-ftello.c (main): Likewise.
* tests/test-nanosleep.c (main): Avoid declaration warning.
* tests/test-sleep.c (main): Likewise.
* tests/test-unsetenv.c (main): Silence warning about string
literal.
* m4/setenv.m4 (gl_FUNC_UNSETENV): Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>