compiler: move tests from vm/tests

These don't belong to VM as they compile some Go code and run it in a VM. One
may call them integration tests, but I prefer to attribute them to
compiler. Moving these tests into pkg/compiler also allows to properly count
the compiler coverage they add:

-ok     github.com/CityOfZion/neo-go/pkg/compiler       (cached)        coverage: 69.7% of statements
+ok     github.com/CityOfZion/neo-go/pkg/compiler       (cached)        coverage: 84.2% of statements

This change also fixes `contant` typo and removes fake packages exposed to the
public by moving foo/bar/foobar into the testdata directory.

pkg/compiler/syscall_test.go

@@ -0,0 +1,22 @@
+package compiler_test
+
+import (
+	"testing"
+)
+
+func TestStoragePutGet(t *testing.T) {
+	src := `
+		package foo
+
+		import "github.com/CityOfZion/neo-go/pkg/interop/storage"
+
+		func Main() string {
+			ctx := storage.GetContext()
+			key := []byte("token")
+			storage.Put(ctx, key, []byte("foo"))
+			x := storage.Get(ctx, key)
+			return x.(string)
+		}
+	`
+	eval(t, src, []byte("foo"))
+}