Fix 'rep_len' for vector string case and 'substring'

...com/oracle/truffle/r/nodes/builtin/base/RepeatLength.java

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

...le.r.test/src/com/oracle/truffle/r/test/all/AllTests.java

...t/src/com/oracle/truffle/r/test/failing/FailingTests.java

.../com/oracle/truffle/r/test/simple/TestSimpleBuiltins.java