Re-enable merge-mode config option and add more tests

This re-enables the ability for projects to define their own merging
behavior via the project merge-mode option.

It re-enables and reworks the old v2.5 test_build_configuration test, which
was written against the cherry-pick merge mode.  Prior to this patch, all
merges were hard-coded to use the merge-recovery mode, so two new tests are
also added here to test those configurations as well.

Change-Id: Ief9430b3a3c33b0c9d7ebf3fd0361166e2a5a1af
Story: 2000785
Task: 3309
diff --git a/tests/fixtures/config/merge-modes/main.yaml b/tests/fixtures/config/merge-modes/main.yaml
new file mode 100644
index 0000000..a22ed5c
--- /dev/null
+++ b/tests/fixtures/config/merge-modes/main.yaml
@@ -0,0 +1,6 @@
+- tenant:
+    name: tenant-one
+    source:
+      gerrit:
+        config-repos:
+          - common-config