tests: Put python dependencies in requirements file

Really this is just `pytest` right now (with `flake8` included for lack
of a better location), but this provides a place to collect more test
dependencies that's better than updating a bunch of adhoc instructions.

https://phabricator.endlessm.com/T34697

.github/workflows/tests.yml

@@ -32,11 +32,11 @@ jobs:
             ostree \
             python3-gi \
             python3-pip
-      - name: Python dependencies
-        run: |
-          python3 -m pip install flake8 pytest
       - name: Checkout
         uses: actions/checkout@v2
+      - name: Python dependencies
+        run: |
+          python3 -m pip install -r requirements-test.txt
       - name: Lint
         run: |
           python3 -m flake8

README.md

@@ -394,8 +394,9 @@ Testing
 =======
 
 Some parts of the image builder can be tested with [pytest][pytest-url].
-After installing pytest, run `pytest` (or `pytest-3` if `pytest` is for
-python 2) from the root of the checkout.
+Install the testing dependencies with `pip3 install -r
+requirements-test.txt`. After installing pytest, run `pytest` (or
+`pytest-3` if `pytest` is for python 2) from the root of the checkout.
 
 Various options can be passed to `pytest` to control how the tests are
 run. See the pytest [usage][pytest-usage] documentation for details.

requirements-test.txt

@@ -0,0 +1,3 @@
+# Python requirements for testing
+flake8
+pytest