Fix backward incompatibility when searching for tests

When locating test programs, try a suffix (-t, .t, or no suffix) with
all bases before moving on to the next suffix.  The behavior in the
previous release was not backward-compatible: it would find the
unsuffixed helper program in the build directory instead of the actual
test in the source directory for some rra-c-util tests when the build
directory and the source directory weren't the same.
Document decision to ignore TAP version directives

These are currently treated as unknown lines and ignored.  This seems
to be correct for now; the current TAP output format doesn't use them,
and the right approach for any future version of TAP would seem to be
to try to parse it anyway.  That ends up being equivalent to ignoring
the version line.
Release 2.1