tests: pin pytest-forked to 1.2.0

The pytest-forked 1.3.0 release isn't compatible with the pytest release
we are using in that branch.

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pytest-forked 1.3.0 requires pytest>=3.10, but you'll have pytest 3.6.1
which is incompatible.
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Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>

tests/requirements.txt

 # These are Python requirements needed to run the functional tests
 six==1.10.0
 testinfra==3.4.0
+pytest-forked==1.2.0
 pytest-xdist==1.27.0
 pytest==3.6.1
 ansible~=2.6,<2.7