Tests: fix requirements for unit tests
Tests: fix requirements for unit tests

Add WebTest to test-requirements which used to be imported as a
transitive requirement via pecan, but the latest release of
pecan dropped this dependency. So make this requirement explicit.

Change-Id: I76bb39f8e247bd7dae4a942f8842c30f5728a3d7
Related-Bug: #1982110

openstack/networking-bgpvpn,openstack/networking-bgpvpn,openstack/networking-bgpvpn