has_monotonic_positions() wants to check for a greater-than-or-equal-to
relation, but actually tests for not-equal, since it treats a
trichotomic comparator as a less-than comparator. This is clearly seen
in the BOOST_FAIL message just below.
Fix by aligning the test with the intended invariant. Luckily, the tests
still pass.
Ref #1449.
Closes#8222