test-version-etc.sh: don't use diff directly: use init.sh's compare
We'd rather not sacrifice readable "diff -u" output even for
"diff -c" output (not supported by busybox) or for even less
readable ed-style "diff" output. So use init.sh's compare function
* tests/test-version-etc.sh: Source init.sh and add "." to path.
Remove "./" from invocation of test-version-etc, so we use path.
And s/diff/compare/.
* modules/version-etc-tests (Depends-on): Add test-framework-sh,
to get init.sh.
Prompted by Eric Blake's comments in
https://lists.gnu.org/r/sed-devel/2018-03/msg00015.html