test: refactor boot test

Use the unittest module from the standard library. Also, ensure that
separate runs of this test don't share a osbuild store and clean up
after themselves.

With contributions from Ondřej Budai and Tom Gundersen.

test/osbuildtest.py

@@ -0,0 +1,21 @@
+
+import json
+import os
+import subprocess
+import sys
+import unittest
+
+
+class TestCase(unittest.TestCase):
+    def run_osbuild(self, pipeline, store):
+            osbuild_cmd = ["python3", "-m", "osbuild", "--json", "--store", store, "--libdir", ".", pipeline]
+
+            build_pipeline = os.getenv("OSBUILD_TEST_BUILD_PIPELINE", None)
+            if build_pipeline:
+                osbuild_cmd.append("--build-pipeline")
+                osbuild_cmd.append(build_pipeline)
+
+            r = subprocess.run(osbuild_cmd, encoding="utf-8", stdout=subprocess.PIPE, check=True)
+
+            result = json.loads(r.stdout)
+            return result["tree_id"], result["output_id"]