docs/devel: Explain how acceptance tests can be skipped

Documented under the "Acceptance tests using the Avocado Framework"
section in testing.rst how environment variables are used to skip tests.

Signed-off-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20210115210022.417996-1-wainersm@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

docs/devel/testing.rst

@@ -871,6 +871,68 @@ qemu_bin
 
 The exact QEMU binary to be used on QEMUMachine.
 
+Skipping tests
+--------------
+The Avocado framework provides Python decorators which allow for easily skip
+tests running under certain conditions. For example, on the lack of a binary
+on the test system or when the running environment is a CI system. For further
+information about those decorators, please refer to::
+
+  https://avocado-framework.readthedocs.io/en/latest/guides/writer/chapters/writing.html#skipping-tests
+
+While the conditions for skipping tests are often specifics of each one, there
+are recurring scenarios identified by the QEMU developers and the use of
+environment variables became a kind of standard way to enable/disable tests.
+
+Here is a list of the most used variables:
+
+AVOCADO_ALLOW_LARGE_STORAGE
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Tests which are going to fetch or produce assets considered *large* are not
+going to run unless that `AVOCADO_ALLOW_LARGE_STORAGE=1` is exported on
+the environment.
+
+The definition of *large* is a bit arbitrary here, but it usually means an
+asset which occupies at least 1GB of size on disk when uncompressed.
+
+AVOCADO_ALLOW_UNTRUSTED_CODE
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+There are tests which will boot a kernel image or firmware that can be
+considered not safe to run on the developer's workstation, thus they are
+skipped by default. The definition of *not safe* is also arbitrary but
+usually it means a blob which either its source or build process aren't
+public available.
+
+You should export `AVOCADO_ALLOW_UNTRUSTED_CODE=1` on the environment in
+order to allow tests which make use of those kind of assets.
+
+AVOCADO_TIMEOUT_EXPECTED
+~~~~~~~~~~~~~~~~~~~~~~~~
+The Avocado framework has a timeout mechanism which interrupts tests to avoid the
+test suite of getting stuck. The timeout value can be set via test parameter or
+property defined in the test class, for further details::
+
+  https://avocado-framework.readthedocs.io/en/latest/guides/writer/chapters/writing.html#setting-a-test-timeout
+
+Even though the timeout can be set by the test developer, there are some tests
+that may not have a well-defined limit of time to finish under certain
+conditions. For example, tests that take longer to execute when QEMU is
+compiled with debug flags. Therefore, the `AVOCADO_TIMEOUT_EXPECTED` variable
+has been used to determine whether those tests should run or not.
+
+GITLAB_CI
+~~~~~~~~~
+A number of tests are flagged to not run on the GitLab CI. Usually because
+they proved to the flaky or there are constraints on the CI environment which
+would make them fail. If you encounter a similar situation then use that
+variable as shown on the code snippet below to skip the test:
+
+.. code::
+
+  @skipIf(os.getenv('GITLAB_CI'), 'Running on GitLab')
+  def test(self):
+      do_something()
+
 Uninstalling Avocado
 --------------------