The testcase iterator::workdir::filesystem_gunk sets up quite a lot of
directories, which is why it only runs in case GITTEST_INVASIVE_SPEED is
set in the environment. Because we do not run our default CI with this
variable, we didn't notice commit 852c83ee (refs: refuse to delete
HEAD, 2020-01-15) breaking the test as it introduced a new reference to
the "testrepo" repository.

Fix the oversight by increasing the number of expected iterator items.