Fix BoringSSL external test failures

We recently turned on the TLSv1.3 downgrade sentinels by default.
Unfortunately we are using a very old version of the BoringSSL test
runner which uses an old draft implementation of TLSv1.3 that also
uses the downgrade sentinels by default. The two implementations do
not play well together and were causing spurious test failures. Until
such time as we update the BoringSSL test runner we disable the failing
tests:

SendFallbackSCSV

In this test the client is OpenSSL and the server is the boring test runner.
The client and server fail to negotiate TLSv1.3 because the test runner is
using an old draft TLSv1.3 version. The server does however add the
TLSv1.3->TLSv1.2 downgrade sentinel in the ServerHello random. Since we
recently turned on checking of the downgrade sentinels on the client side
this causes the connection to fail.

VersionNegotiationExtension-TLS11

In this test the test runner is the client and OpenSSL is the server. The
test modifies the supported_versio...