Add back a few tests about Holes

src/test/resources/regression/synthesis/Holes/Hole1.scala

+import leon.lang._
+import leon.collection._
+import leon.lang.synthesis._
+
+object Holes {
+  def abs1(a: Int) = {
+    if (?(true, false)) {
+      a
+    } else {
+      -a
+    }
+  } ensuring {
+    _ >= 0
+  }
+
+  def abs2(a: Int) = {
+    if (???[Boolean]) {
+      a
+    } else {
+      -a
+    }
+  } ensuring {
+    _ >= 0
+  }
+
+  def abs3(a: Int) = {
+    if (?(a > 0, a < 0)) {
+      a
+    } else {
+      -a
+    }
+  } ensuring {
+    _ >= 0
+  }
+}