Modifications in FastR to be able to install and run package "testthat".

"testthat" has been tested running tests of package "argparse".

- implemented RFFI functions PRSEEN, PRENV, R_PromiseExpr, and PRCODE
- cleanup of method "getIntrinsicValue"; replaced by RASTUtils.createLanguageElement
- added dummy cast to node IsATTY
- also fixed FileConnections.writeString; was required to run a package
- added tests for R exception handling; since we still have bugs, some tests are ignored
- also fixed a problem during source code parsing in REngine, which occurred if the code contained empty lines that have not been suppressed by any shell