Refactor individual run-test build scripts

Follow the same convention as run-test run scripts.

The python files shall export build function,
rather than doing the work when the script is loaded.

This has the advantage that we now pass context/arguments
to the function (rather than using environment variables).

Test: the generated build artefacts are identical
Change-Id: I90e0ef0d2e31b27813042d51d07b5ae132e1e704
diff --git a/test/663-odd-dex-size2/build.py b/test/663-odd-dex-size2/build.py
index 76845c9..f304d95 100644
--- a/test/663-odd-dex-size2/build.py
+++ b/test/663-odd-dex-size2/build.py
@@ -13,4 +13,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-# Nothing to do.
+
+def build(ctx):
+  pass  # Nothing to do.