Make rednet.run a little more strict

Also add a test for rednet message sending. Hopefully gives some of the
modem and networking code a little bit of coverage (which is clearly the
same as being right :p).

src/test/java/dan200/computercraft/ingame/CraftOsTest.kt

@@ -0,0 +1,13 @@
+package dan200.computercraft.ingame
+
+import dan200.computercraft.ingame.api.GameTest
+import dan200.computercraft.ingame.api.TestContext
+import dan200.computercraft.ingame.api.checkComputerOk
+
+class CraftOsTest {
+    /**
+     * Sends a rednet message to another a computer and back again.
+     */
+    @GameTest
+    suspend fun `Sends basic rednet messages`(context: TestContext) = context.checkComputerOk(13)
+}