don't let a fail=1 env. setting induce unwarranted test failure

[Jim Meyering]

I've just pushed these four:

  tests: don't let a fail=1 env. setting induce unwarranted test failure
  tests: remove the less-regular fail=0 assignments manually
  tests: factor 350 fail=0 initializations into test-lib.sh
  tests: prohibit fail=0 initialization

I preferred not to squash into the initial one because there was some
manual fix-up in there (lesson: don't do that).
Hence, the latter three are more segregated.
The only disadvantage is that an intermediate commit or two will
probably fail a syntax check.  However, I have never used git bisect
with the syntax-check rule, so that should be ok.

Interesting fall-out was that "make fail=1" would fail in
any project with a subdirectory, due to an automake bug fixed here:

    http://thread.gmane.org/gmane.comp.sysutils.automake.patches/3724