frontend xlang tests moved to their own package

src/test/resources/regression/xlang/error/Array1.scala

src/test/resources/regression/xlang/error/Array10.scala

src/test/resources/regression/xlang/error/Array2.scala

src/test/resources/regression/xlang/error/Array3.scala

src/test/resources/regression/xlang/error/Array4.scala

src/test/resources/regression/xlang/error/Array5.scala

src/test/resources/regression/xlang/error/Array6.scala

src/test/resources/regression/xlang/error/Array7.scala

src/test/resources/regression/xlang/error/Array8.scala

src/test/resources/regression/xlang/error/Array9.scala

...test/scala/leon/regression/frontends/FrontEndsSuite.scala