tests/avocado: mark boot_linux.py long runtime instead of flaky

The ppc64 and s390x tests were first marked skipIf GITLAB_CI by commit
c0c8687ef0f ("tests/avocado: disable BootLinuxPPC64 test in CI"), and
commit 0f26d94ec9e ("tests/acceptance: skip s390x_ccw_vrtio_tcg on
GitLab") due to being very heavy-weight for gitlab CI.

Commit 9b45cc99318 ("docs/devel: rationalise unstable gitlab tests under
FLAKY_TESTS") changed this to being flaky but it isn't really, it just
had a long runtime.

So take the SPEED=slow variable from qtests and introduce it to avocado,
and make these tests require it.

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

docs/devel/testing.rst

@@ -1346,6 +1346,17 @@ 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.
 
+SPEED
+^^^^^
+Tests which have a long runtime will not be run unless ``SPEED=slow`` is
+exported on the environment.
+
+The definition of *long* is a bit arbitrary here, and it depends on the
+usefulness of the test too. A unique test is worth spending more time on,
+small variations on existing tests perhaps less so. As a rough guide,
+a test or set of similar tests which take more than 100 seconds to
+complete.
+
 AVOCADO_ALLOW_UNTRUSTED_CODE
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 There are tests which will boot a kernel image or firmware that can be

tests/avocado/boot_linux.py

@@ -93,13 +93,11 @@ class BootLinuxPPC64(LinuxTest):
 
     timeout = 360
 
-    @skipUnless(os.getenv('QEMU_TEST_FLAKY_TESTS'), 'Test is unstable on GitLab')
-
+    @skipUnless(os.getenv('SPEED') == 'slow', 'runtime limited')
     def test_pseries_tcg(self):
         """
         :avocado: tags=machine:pseries
         :avocado: tags=accel:tcg
-        :avocado: tags=flaky
         """
         self.require_accelerator("tcg")
         self.vm.add_args("-accel", "tcg")
@@ -113,13 +111,11 @@ class BootLinuxS390X(LinuxTest):
 
     timeout = 240
 
-    @skipUnless(os.getenv('QEMU_TEST_FLAKY_TESTS'), 'Test is unstable on GitLab')
-
+    @skipUnless(os.getenv('SPEED') == 'slow', 'runtime limited')
     def test_s390_ccw_virtio_tcg(self):
         """
         :avocado: tags=machine:s390-ccw-virtio
         :avocado: tags=accel:tcg
-        :avocado: tags=flaky
         """
         self.require_accelerator("tcg")
         self.vm.add_args("-accel", "tcg")