We were setting a global variable FAIL on failure, but if we failed
inside a subshell (which a very small number of tests might do), this
setting would be lost. The script output (a series of failed/warning
lines) was still valid, but not the return code, so the shell might be
selected even if one of these tests failed.
To avoid the problem, put the fail/warning state in the filesystem
instead, which is shared across subshells.
