tests(scalar): tighten the stale `scalar.repo` test some

tests(scalar): tighten the stale `scalar.repo` test some

As pointed out by Stolee, the previous incarnation of this test case was
not stringent enough: we want to verify that _only_ the stale entries
are removed (previously, the test case would have succeeded even if all
entries had been removed).

Let's rectify this and verify that the other entries are left intact.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Taylor Blau <me@ttaylorr.com>