Add tests for passing args to run scripts (#647)

## Summary
TSIA, pretty straight-forward.

## How was it tested?
```
> go test -run TestScripts/run/args.test ./testscripts/testscripts_test.go
```

Author: ipince

testscripts/run/args.test.txt

@@ -0,0 +1,13 @@
+# Test passing arguments to a script
+exec devbox run ekko hello there
+stdout 'hello there'
+
+-- devbox.json --
+{
+  "packages": [],
+  "shell": {
+    "scripts": {
+      "ekko": "echo $@"
+    }
+  }
+}