tests: factor 350 fail=0 initializations into test-lib.sh

Run this command to remove the factored-out "fail=0" lines.
perl -ni -e '/^fail=0$/ or print' $(g grep -l '^fail=0$')
* tests/test-lib.sh: Initialize fail=0 here, not in 300+ scripts.
* tests/...: nearly all bourne shell scripts
Suggested by Eric Blake.

tests/id/no-context

@@ -27,7 +27,6 @@ fi
 # but this is as good a witness as any, in general.
 require_selinux_
 
-fail=0
 
 # Require the context=... part by default.
 id > out || fail=1