Fixed sandbox.js

There was a bug that didn't allow you to run execute multiple times, especially after something like a delay. A flag was being reset. Being able to run .execute multiple times is part of the charm.

Includes a test.
Run with "node fix2.js"

vanesca-tests/fix2.js

@@ -0,0 +1,56 @@
+
+process.stdout.write("\u001b[2J\u001b[00H") // clear screen
+const { Sandbox, SandboxCluster } = require('../v8-sandbox')
+let sandbox = new Sandbox({ memory: 1000 })
+
+let code1 = `
+    let t = 1
+    console.log(t) // works
+`; // always end with semicolon
+
+let code2 = ` // repeated executes STACK and should keep their values
+    t++
+    console.log(t)
+`;
+
+(async () => {
+
+    const RUN = 4
+    console.log('Test no.' + RUN)
+
+    // test 1 (runs in sync) (works)
+    if(RUN === 1) {
+        sandbox.execute({ code: code1 })
+        sandbox.execute({ code: code2 })
+    }
+
+    //////////////////////////////////////////
+
+    // test 2 (runs first in async/second sync) (doesn't work, even though it should!)
+    if(RUN === 2) {
+        await sandbox.execute({ code: code1 })
+        sandbox.execute({ code: code2 })
+    }
+
+    //////////////////////////////////////////
+
+    // test 3 (run in async) (doesn't work, even though it should!)
+    if(RUN === 3) {
+        await sandbox.execute({ code: code1 })
+        await sandbox.execute({ code: code2 })
+    }
+
+    //////////////////////////////////////////
+
+    // test 4 (promise result also doesn't work, even though it should!)
+    if(RUN === 4) {
+        sandbox.execute({ code: code1 }).then(() =>
+            // doesn't work, even though it should!
+            // because of a flag being reset in sandbox.js code
+            // this is fixed now.
+            setTimeout(async() => {
+               sandbox.execute({ code: code2 }) // should display '2' (after fix)
+            }, 2000)
+        )
+    }
+})()