MFC r315032-r315036, r315039, r315041

r315032:
Increase WARNS for iconv tests

ATF tests have a default WARNS of 0, unlike other usermode programs.

Reviewed by: ngie
Sponsored by: Spectra Logic Corporation
Differential Revision: https://reviews.freebsd.org/D9933

r315033:
Increase WARNS for nss tests

ATF tests have a default WARNS of 0, unlike other usermode programs.

Reviewed by: ngie
Sponsored by: Spectra Logic Corporation
Differential Revision: https://reviews.freebsd.org/D9933

r315034:
Document that the msun tests require WARNS=0

ATF tests have a default WARNS of 0, unlike other usermode programs. This
change is technically a noop, but it documents that the msun tests don't
work with any warnings enabled, at least not on all architectures.

Reviewed by: ngie
Sponsored by: Spectra Logic Corporation
Differential Revision: https://reviews.freebsd.org/D9933

r315035:
Increase WARNS for libcrypt tests

ATF tests have a default WARNS of 0, unlike other usermode programs.

Reviewed by: ngie, julian
Sponsored by: Spectra Logic Corporation
Differential Revision: https://reviews.freebsd.org/D9933

r315036:
Increase WARNS for libmp tests

ATF tests have a default WARNS of 0, unlike other usermode programs.

Reviewed by: ngie, julian
Sponsored by: Spectra Logic Corporation
Differential Revision: https://reviews.freebsd.org/D9933

r315039:
Increase WARNS for libutil tests

ATF tests have a default WARNS of 0, unlike other usermode programs.

Reviewed by: ngie, julian
Sponsored by: Spectra Logic Corporation
Differential Revision: https://reviews.freebsd.org/D9933

r315041:
Increase WARNS for pw tests

ATF tests have a default WARNS of 0, unlike other usermode programs.

Reviewed by: ngie, julian
Sponsored by: Spectra Logic Corporation
Differential Revision: https://reviews.freebsd.org/D9933