pybind/mgr/dashboard: move pytest into requirements.txt

pybind/mgr/dashboard: move pytest into requirements.txt

before this change, pytest is included by both requirements-lint.txt
and requirements-test.txt. this fails the install-deps.sh script when
collecting the python package wheels:

ERROR: Double requirement given: pytest<4 (from -r requirements-test.txt (line 2)) (already in pytest (from -r requirements-lint.txt (line 12)), name='pytest')

also, since pytest is unconditionally imported in the source, for instance,
in pybind/mgr/dashboard/tests/test_ceph_service.py

it would be more straightforward just to include it in requirements.txt.

Signed-off-by: Kefu Chai <kchai@redhat.com>