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/constant_test.go

@@ -0,0 +1,63 @@
+package compiler_test
+
+import (
+	"math/big"
+	"testing"
+)
+
+func TestBasicConstant(t *testing.T) {
+	src := `
+		package foo
+
+		const x = 10
+
+		func Main() int {
+			return x + 2
+		}
+	`
+	eval(t, src, big.NewInt(12))
+}
+
+func TestShortHandMultiConst(t *testing.T) {
+	src := `
+		package foo
+
+		const (
+			z = 3
+			y = 2
+			x = 1
+		)
+
+		// should load al 3 constants in the Main.
+		func Main() int {
+			return x + z + y
+		}
+	`
+	eval(t, src, big.NewInt(6))
+}
+
+func TestGlobalsWithFunctionParams(t *testing.T) {
+	src := `
+		package foobar
+
+		const (
+			// complex he o_O
+			bar = "FOO" 
+			foo = "BAR"
+		)
+
+		func something(x int) string {
+			if x > 10 {
+				return bar
+			}
+			return foo
+		}
+
+		func Main() string {
+			trigger := 100
+			x := something(trigger)
+			return x
+		}
+	`
+	eval(t, src, []byte("FOO"))
+}