We currently lack coverage for unencrypted PKCS#8 private keys (-----BEGIN PRIVATE KEY-----) with the sole exception of test/fixtures/foafssl.key used in test/parallel/test-https-foafssl.js.

Compare PKCS#1 private keys (-----BEGIN RSA PRIVATE KEY-----) for which we have ample coverage.

I count three tests that cover encrypted PKCS#8 keys (-----BEGIN ENCRYPTED PRIVATE KEY-----):

test/parallel/test-tls-passphrase.js
test/parallel/test-crypto-rsa-dsa.js
test/parallel/test-crypto-rsa-dsa.js

For unencrypted PKCS#8, it would be nice to have tests for at least RSA and DSA, with and without PEM attributes for bonus points. See test/fixtures/foafssl.key for an example of attributes.

cc @sam-github - possibly relevant to your interests