Support declaring some test directives from the main scope

[Gedochao]

Fixes #1729

Some select directives (and not others) can now be declared for the test scope even in main scope sources.
Those now support a separate key with the test.* prefix, indicating the test scope target.
The recommended location for declaring such directives would be the optional project.scala configuration file.

Directives supporting this feature:

//> using test.jar "<custom jar path>"
//> using test.dep "<dependency>"
//> using test.option "<scalac option>"
//> using test.javacOpt "<javac option>"
//> using test.javaOpt "<java option>"
//> using test.javaProp "<java properties>"
//> using test.toolkit "latest"
//> using test.resourceDir "<resources directory>"

[lwronski]

Add explanation in the directive guide about the test. prefix, and include a list of the available options.

[Gedochao]

Add explanation in the directive guide about the test. prefix, and include a list of the available options.

@lwronski I'll do a followup PR with refactorings and docs, don't want to make this one bigger