Move Symbol to shared together with jvm specific domain tests.

[mikla]

Closes #261

• Symbol was moved to domain/shared
• Decided to move all domain/jvm test to domain/shared. So they are run agains JS and JVM right now.
• Added .sbtopts as sbt test started to fail with OOM.

Hope I didn't break anything. 🤞

[gvolpe]

Hey @mikla thanks for doing this. I won't be merging it, though, because I want the main branch to reflect what's in the book. Nevertheless, I think this is a good reference for everyone now that Iron supports Scala.js.