selftests: ovpn: allow compiling ovpn-cli.c with mbedtls3

selftests: ovpn: allow compiling ovpn-cli.c with mbedtls3

mbedtls 3 installs headers and calls the shared object
differently than version 2, therefore we must now rely
on pkgconfig to fill the right C/LDFLAGS.

Moreover the mbedtls3 library expects any base64 file to
have their content on one line.
Since this change does no break older versions,
let's change the sample key file format and make mbedtls3
happy.

Cc: Shuah Khan <shuah@kernel.org>
Cc: linux-kselftest@vger.kernel.org
Cc: horms@kernel.org
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>