Add regression tests for C code generation

build.sbt

src/test/resources/regression/genc/invalid/AbsFun.scala

...test/resources/regression/genc/invalid/LinearSearch.scala

src/test/resources/regression/genc/valid/BinarySearch.scala

...est/resources/regression/genc/valid/BinarySearchFun.scala

...est/resources/regression/genc/valid/ExpressionOrder.scala

src/test/resources/regression/genc/valid/MaxSum.scala

...s/regression/genc/valid/RecursionAndNestedFunctions.scala

src/test/resources/regression/genc/valid/TupleArray.scala

src/test/scala/leon/genc/GenCSuite.scala