Tests: Update .gitreview

Use main branch by default.

Change-Id: I5fc5e6ed3010b1c07cc99ea8698aaaf68cdc2b91
diff --git a/.gitreview b/.gitreview
new file mode 100644
index 0000000..226af35
--- /dev/null
+++ b/.gitreview
@@ -0,0 +1,5 @@
+[gerrit]
+host=review.trustedfirmware.org
+port=29418
+project=TF-M/tf-m-tests
+defaultbranch=main