Using test files from the released jar is brittle and can stop a bootstrap

[jurgenvinju]

Describe the bug

In many compiler tests we compile modules from |std:///| to see how the compiler is doing on the standard library.

1. This is still a good idea, but currently we are reading those sources from jar files which are already released and immutable. They are guaranteed to come from <bootstrapRascalVersion>0.41.0-RC34</bootstrapRascalVersion> in the pom.xml because that is what |std:///| resolves to.
2. If rascal evolves and the object code under test needs adaptation, there is currently no way of doing that.
3. Propose to change |std:///| to |project://rascal/src/org/rascalmpl/library|
4. We can also make an alias for that or at least a global variable to share this location among the tests.