Fix test spec.5 to close inappbrowser after loadstop event

[knight9999]

Platforms affected

all platforms (but test only)

Motivation and Context

The spec 'inappbrowser.spec.5 should support exit event' in cordova-plugin-inappbrowser-tests.tests is testing exit event.
But before this PR, close method may be fired before InAppBrowser opens a URL.
Then cordova-windows, the corresponding test (spec.6) sometimes fails, the InAppBrowser is kept opened.
Then the next spec (spec.6) fails.

This PR is to fix this issue.

Description

Instead of calling close() method synchronously,
I call close() method in callback of loadstop event.

Testing

With the help of cordova-mobile-spec, I confirmed all cordova-plugin-inappbrowser-tests.tests work well.

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary