README: mention that openssl and polarssl will be used for interop testing

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  Test
failed #8267
Build and Check

01:02:57

 
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make[3]: *** [check-TESTS] Error 2
make[3]: Leaving directory '/home/gitlab_ci_runner/gitlab-ci-runner/tmp/builds/project-684/tests/cert-tests'
Makefile:1724: recipe for target 'check-am' failed
make[2]: *** [check-am] Error 2
make[2]: Leaving directory '/home/gitlab_ci_runner/gitlab-ci-runner/tmp/builds/project-684/tests/cert-tests'
Makefile:2953: recipe for target 'check-recursive' failed
make[1]: *** [check-recursive] Error 1
make[1]: Leaving directory '/home/gitlab_ci_runner/gitlab-ci-runner/tmp/builds/project-684/tests'
Makefile:1388: recipe for target 'check-recursive' failed
make: *** [check-recursive] Error 1