Write changelog entry for the test data update

We normally don't mention test updates in the changelog, but this one
explains why we're making a patch release.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/ChangeLog.d/test-data-202308.txt b/ChangeLog.d/test-data-202308.txt
new file mode 100644
index 0000000..51dbd18
--- /dev/null
+++ b/ChangeLog.d/test-data-202308.txt
@@ -0,0 +1,2 @@
+Changes
+   * Update test data to avoid failures of unit tests after 2023-08-07.