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/byte_conversion_test.go

@@ -0,0 +1,56 @@
+package compiler_test
+
+import "testing"
+
+func TestStringToByteConversion(t *testing.T) {
+	src := `
+	package foo
+	func Main() []byte {
+		b := []byte("foo")
+		return b
+	}
+	`
+	eval(t, src, []byte("foo"))
+}
+
+func TestStringToByteAppend(t *testing.T) {
+	src := `
+	package foo
+	func Main() []byte {
+		b := []byte("foo")
+		c := []byte("bar")
+		e := append(b, c...)
+		return e
+	}
+	`
+	eval(t, src, []byte("foobar"))
+}
+
+func TestByteConversionInFunctionCall(t *testing.T) {
+	src := `
+	package foo
+	func Main() []byte {
+		b := []byte("foo")
+		return handle(b)
+	}
+
+	func handle(b []byte) []byte {
+		return b
+	}
+	`
+	eval(t, src, []byte("foo"))
+}
+
+func TestByteConversionDirectlyInFunctionCall(t *testing.T) {
+	src := `
+	package foo
+	func Main() []byte {
+		return handle([]byte("foo"))
+	}
+
+	func handle(b []byte) []byte {
+		return b
+	}
+	`
+	eval(t, src, []byte("foo"))
+}