Add tests for verify_restartable()

For selection of test cases, see comments added in the commit.

It makes the most sense to test with chains using ECC only, so for the chain
of length 2 we use server10 -> int-ca3 -> int-ca2 and trust int-ca2 directly.

Note: server10.crt was created by copying server10_int3_int-ca2.crt and
manually truncating it to remove the intermediates. That base can now be used
to create derived certs (without or with a chain) in a programmatic way.
diff --git a/tests/data_files/server10.crt b/tests/data_files/server10.crt
new file mode 100644
index 0000000..96a4040
--- /dev/null
+++ b/tests/data_files/server10.crt
@@ -0,0 +1,10 @@
+-----BEGIN CERTIFICATE-----
+MIIBWjCCAQCgAwIBAgIBSzAKBggqhkjOPQQDAjBKMQswCQYDVQQGEwJVSzERMA8G
+A1UEChMIbWJlZCBUTFMxKDAmBgNVBAMTH21iZWQgVExTIFRlc3QgaW50ZXJtZWRp
+YXRlIENBIDMwHhcNMTUwOTAxMTM0NzU1WhcNMjUwODI5MTM0NzU1WjAUMRIwEAYD
+VQQDEwlsb2NhbGhvc3QwWTATBgcqhkjOPQIBBggqhkjOPQMBBwNCAAQcbffp2qXq
+oZyychmoCRxzrd4Vu96m47NPBehtEC46aTeXgDnBdf++znABrAtfXBRNQz8ARIeY
+Bmskr22rlKjyow0wCzAJBgNVHRMEAjAAMAoGCCqGSM49BAMCA0gAMEUCIQDLc+Io
+rg8VxEbCgVv8iH+kOIEn9MjhpvKzvwUoV+6rjQIgZU/RXAyc1a+H2+soGfNEIOBQ
+AzO3pJx7WJAApZuBX1Q=
+-----END CERTIFICATE-----