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/du/basic

@@ -32,7 +32,6 @@ printf '%*s' 257 make-sure-the-file-is-non-empty > a/b/F || framework_failure
 printf %4096s x > d/1
 cp d/1 d/sub/2
 
-fail=0
 
 B=`stat --format=%B a/b/F`