Speed up test_crossencoder

[stephenroller]

Patch description
Speed up one of our slower, flaky tests. As far as I can tell this is about 2x faster and doesn't fail (while having a higher threshold for accuracy lol). Major change is switching to an overfit dataset.

Testing steps
New CI and local pytest runs.

[stephenroller]

In this commit:

146.22s call     tests/nightly/gpu/test_bert.py::TestBertModel::test_biencoder
81.06s call     tests/nightly/gpu/test_bert.py::TestBertModel::test_crossencoder

In a recent master commit:

466.35s call     tests/nightly/gpu/test_bert.py::TestBertModel::test_biencoder
358.52s call     tests/nightly/gpu/test_bert.py::TestBertModel::test_crossencoder