reftable/basics: improve `binsearch()` test

The `binsearch()` test is somewhat weird in that it doesn't explicitly
spell out its expectations. Instead it does so in a rather ad-hoc way
with some hard-to-understand computations.

Refactor the test to spell out the needle as well as expected index for
all testcases. This refactoring highlights that the `binsearch_func()`
is written somewhat weirdly to find the first integer smaller than the
needle, not smaller or equal to it. Adjust the function accordingly.

While at it, rename the callback function to better convey its meaning.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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