Added SHA256 test certificates

With SHA-1 deprecation, we need a few certificates using algorithms in
the default support list. Most tests still use SHA-1 though.

The generation process for the new certificates is recorded in the makefile.
diff --git a/tests/data_files/test-ca-sha256.crt b/tests/data_files/test-ca-sha256.crt
new file mode 100644
index 0000000..b14e405
--- /dev/null
+++ b/tests/data_files/test-ca-sha256.crt
@@ -0,0 +1,21 @@
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----