test: refactor test-tls-two-cas-one-string

• order require() statements per test writing guide
• add keydir variable to make readFileSync() calls more readable
• make next argument to test() optional
• use common.mustCall() to guarantee second test runs

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

test tls