Unable to run unittest that contain ValidationRules

**I'm submitting a bug report**

* **Library Version:**
1.0.0

**Please tell us about your environment:**
* **Operating System:**
Windows 10

* **Node Version:**
V10.16.0

* **NPM Version:**
6.9.0

* **JSPM OR Webpack AND Version**
webpack 4.34.0

* **Browser:**
n/a

* **Language:**
TypeScript 3.1.2

**Current behavior:**
When running a test that tests a component which uses ValidationRules, the following error pops up:

```Error invoking App. Check the inner error for details.
------------------------------------------------
Inner Error:
Message: Unable to parse accessor function:
function (a) {
/* istanbul ignore next */
cov_q8rv1u7dl.f[2]++;
cov_q8rv1u7dl.s[5]++;
return a.message;
}
Inner Error Stack:
Error: Unable to parse accessor function:
function (a) {
/* istanbul ignore next */
cov_q8rv1u7dl.f[2]++;
cov_q8rv1u7dl.s[5]++;
return a.message;
}

at getAccessorExpression (node_modules/aurelia-validation/dist/commonjs/aurelia-validation.js:113:15)
at PropertyAccessorParser.Object..PropertyAccessorParser.parse (node_modules/aurelia-validation/dist/commonjs/aurelia-validation.js:95:28)
at FluentEnsure.Object..FluentEnsure.ensure (node_modules/aurelia-validation/dist/commonjs/aurelia-validation.js:1703:42)
at Function.Object..ValidationRules.ensure (node_modules/aurelia-validation/dist/commonjs/aurelia-validation.js:1770:58)
at new App (src/app.ts:219:42)
at Object.invoke (node_modules/aurelia-dependency-injection/dist/commonjs/aurelia-dependency-injection.js:379:24)
at InvocationHandler.Object..InvocationHandler.invoke (node_modules/aurelia-dependency-injection/dist/commonjs/aurelia-dependency-injection.js:352:28)
at Container.Object..Container.invoke (node_modules/aurelia-dependency-injection/dist/commonjs/aurelia-dependency-injection.js:538:28)
at ProviderResolver.get (node_modules/aurelia-templating/dist/commonjs/aurelia-templating.js:2305:72)
at Container.Object..Container.get (node_modules/aurelia-dependency-injection/dist/commonjs/aurelia-dependency-injection.js:493:28)
End Inner Error Stack
------------------------------------------------
at new AggregateError (node_modules/aurelia-pal/dist/commonjs/aurelia-pal.js:37:11)
at Container.Object..Container.invoke (node_modules/aurelia-dependency-injection/dist/commonjs/aurelia-dependency-injection.js:541:19)
at ProviderResolver.get (node_modules/aurelia-templating/dist/commonjs/aurelia-templating.js:2305:72)
at Container.Object..Container.get (node_modules/aurelia-dependency-injection/dist/commonjs/aurelia-dependency-injection.js:493:28)
at Container.elementContainerGet [as get] (node_modules/aurelia-templating/dist/commonjs/aurelia-templating.js:2360:15)
at HtmlBehaviorResource.create (node_modules/aurelia-templating/dist/commonjs/aurelia-templating.js:4379:56)
at applyInstructions (node_modules/aurelia-templating/dist/commonjs/aurelia-templating.js:2470:31)
at ViewFactory.create (node_modules/aurelia-templating/dist/commonjs/aurelia-templating.js:2691:7)
at TemplatingEngine.enhance (node_modules/aurelia-templating/dist/commonjs/aurelia-templating.js:5227:24)
at node_modules/aurelia-framework/dist/commonjs/aurelia-framework.js:176:28
```

I have searched for a proper way to unittest this, but I'm unable to find a solution. Hopefully the community can help me out.

**What is the expected behavior?**
A possibility to run unittest that contains ValidationRules without it throwing an exception.

I have created a public repo where you can find all the files necessary: https://github.com/JordyThien/aurelia-unittest-validation-rules
Basically I created a new aurelia project using the aurelia CLI: `au new aurelia-testing --unattended --select typescript,jest,vscode --here`

I've updated the https://github.com/JordyThien/aurelia-unittest-validation-rules/blob/master/src/app.ts to contain ValidationRules
And I updated the spec to add `aurelia-validation`
Where running the tests using `au test` the exception will be thrown.

Start with src/app.ts and the updated spec in the linked reproduction repository, then run `au test` to reproduce the ValidationRules failure. Trace the reported call into aurelia-validation.js and compare the test setup with the application setup. Done means the component test runs without the accessor parsing exception.

Written by the indexing model from the issue text.