tests/functional: purge scratch dir on test startup

The test suite purges the scratch dir in the tearDown method, but
if python crashes (or is non-gracefully killed) this won't get run.
Also the user can set QEMU_TEST_KEEP_SCRATCH to disable cleanup.

Purging the scratch dir on startup ensures that tests always run
from a clean state.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-ID: <20250908135722.3375580-5-berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

tests/functional/qemu_test/testcase.py

@@ -205,6 +205,10 @@ class QemuBaseTest(unittest.TestCase):
         self.outputdir = self.build_file('tests', 'functional',
                                          self.arch, self.id())
         self.workdir = os.path.join(self.outputdir, 'scratch')
+        if os.path.exists(self.workdir):
+            # Purge as safety net in case of unclean termination of
+            # previous test, or use of QEMU_TEST_KEEP_SCRATCH
+            shutil.rmtree(self.workdir)
         os.makedirs(self.workdir, exist_ok=True)
 
         self.log_filename = self.log_file('base.log')