restructure builtin tests

...est/src/com/oracle/truffle/r/test/ExpectedTestOutput.test

...c/com/oracle/truffle/r/test/builtins/TestBuiltin_Arg.java

.../com/oracle/truffle/r/test/builtins/TestBuiltin_Conj.java

...cle/truffle/r/test/builtins/TestBuiltin_Date2POSIXlt.java

.../oracle/truffle/r/test/builtins/TestBuiltin_Encoding.java

.../truffle/r/test/builtins/TestBuiltin_Encodingassign_.java

...acle/truffle/r/test/builtins/TestBuiltin_ISOdatetime.java

...rc/com/oracle/truffle/r/test/builtins/TestBuiltin_Im.java

...rc/com/oracle/truffle/r/test/builtins/TestBuiltin_La.java

...c/com/oracle/truffle/r/test/builtins/TestBuiltin_Mod.java

...cle/truffle/r/test/builtins/TestBuiltin_POSIXlt2Date.java

...oracle/truffle/r/test/builtins/TestBuiltin_Primitive.java

...rc/com/oracle/truffle/r/test/builtins/TestBuiltin_Re.java

...om/oracle/truffle/r/test/builtins/TestBuiltin_Reduce.java

.../oracle/truffle/r/test/builtins/TestBuiltin_Syschmod.java

...oracle/truffle/r/test/builtins/TestBuiltin_Sysgetenv.java

...m/oracle/truffle/r/test/builtins/TestBuiltin_Sysglob.java

...acle/truffle/r/test/builtins/TestBuiltin_Sysreadlink.java

...oracle/truffle/r/test/builtins/TestBuiltin_Syssetenv.java

...cle/truffle/r/test/builtins/TestBuiltin_Syssetlocale.java