vm: fix debugger and add tests

1. `Run()` must be able to continue execution after a breakpoint.
2. VM must stop right before the breakpoint, not after.
3. Initial vm state is NONE, not HALT.

pkg/vm/context.go

@@ -221,7 +221,7 @@ func (c *Context) Equals(s stackitem.Item) bool {
 
 func (c *Context) atBreakPoint() bool {
 	for _, n := range c.breakPoints {
-		if n == c.ip {
+		if n == c.nextip {
 			return true
 		}
 	}