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/numeric_test.go

@@ -0,0 +1,25 @@
+package compiler_test
+
+import (
+	"math/big"
+	"testing"
+)
+
+var numericTestCases = []testCase{
+	{
+		"add",
+		`
+		package foo
+		func Main() int {
+			x := 2
+			y := 4
+			return x + y
+		}
+		`,
+		big.NewInt(6),
+	},
+}
+
+func TestNumericExprs(t *testing.T) {
+	runTestCases(t, numericTestCases)
+}