test/T010-help-test.sh fails after ./configure --without-docs

[Daniel Kahn Gillmor]

On a system where man and notmuch are both already installed, i tried to
./configure --without-docs to avoid the delays during development
associated with id:87r29wwgq2.fsf at fifthhorseman.net.

The configuration succeeds, but now these tests fail:

0 dkg at alice:~/src/notmuch/notmuch$ make check NOTMUCH_TESTS=T010-help-test.sh V=1
Missing sphinx or makeinfo, not building info pages
INFO: using 2 minute timeout for tests

T010-help-test: Testing online help
 PASS   notmuch --help
 PASS   notmuch help
 PASS   notmuch --version
 FAIL   notmuch --help tag (man pages not available)
	test_must_fail notmuch --help tag >/dev/null
 FAIL   notmuch help tag (man pages not available)
	test_must_fail notmuch help tag >/dev/null

Notmuch test suite complete.
3/5 tests passed.
2 tests failed.
make: *** [test/Makefile.local:75: test] Error 1
[1]+  Done                    gitk --all
2 dkg at alice:~/src/notmuch/notmuch$

It seems likely that they're "failing to fail" because the main system
i'm building on has both notmuch and man installed -- so maybe the
locally-built notmuch is successfully opening the manual pages from the
system instead of the development manpages.

I did a bit of digging to try to understand the issue, and noticed that
the comments about test_must_fail in test/test-lib.sh seem out of date
(they assume two arguments instead of one, and they refer to their
earlier provenance in git, rather than notmuch).  But i think the
problem isn't with test_must_fail itself, even though the function's
documentation appears to be out of date.

              --dkg
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