test: use fipsMode in test-crypto-fips

This commit updates test/parallel/test-crypto-fips.js to use fipsMode
instead of process.config.variables.openssl_fips.

The motivation for this is that since the addition of the
--openssl-is-fips configuration flag, it is possible to dynamically
link with a FIPS compliant OpenSSL library. Using fipsMode allows for
determining if there is FIPS support when statically or dynamically
linking against a FIPS compatible OpenSSL library.

PR-URL: https://github.com/nodejs/node/pull/25563


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>