MFC r322677: pw usermod: handle empty secondary group lists (-G '')

"pw usermod someuser -G ''" is supposed make sure that someuser
doesn't have any secondary group memberships.

Previouly it was a nop because split_groups() only intitialised
"groups" if at least one group was specified. As a result the
existing secondary group memberships were kept.

PR: 221417
Submitted by: Fabian Keil
Approved by: re (kib)
Obtained from: ElectroBSD
Relnotes: yes

MFC r315776:

Rename tests from <foo> to <foo>_test to match the FreeBSD test suite
naming scheme

usr.bin/diff/diff_test was renamed to usr.bin/diff/netbsd_diff_test
to avoid collisions with the renamed FreeBSD test.