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/sample-test

@@ -29,7 +29,6 @@ fi
 # FIXME: cleanup_() { rm -rf "$other_partition_tmpdir"; }
 # FIXME: . "$abs_srcdir/other-fs-tmpdir"
 
-fail=0
 FIXME > out || fail=1
 cat <<\EOF > exp || fail=1
 FIXME