test: improve the code in test-crypto-rsa-dsa

• use const and let instead of var
• use assert.strictEqual instead of assert.equal
• use arrow functions
• swap assertions arguments to match the standard

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

test