fix: adds trait_exists to ensure orchestra/testbench is installed

[rajyan]

• Added or updated tests
• Documented user facing changes
• Updated CHANGELOG.md

resolves #863

Related PRs

• fix bootstrap path #852
• Revert "fix bootstrap path (#852)" #858

I personally think getcwd() is not a good solution to
find Laravel Application bootstrap file, but could not find a better solution that solve cases like #856.

so, for cases that Laravel application bootstrap file cannot be loaded by getcwd().'/bootstrap/app.php',
I just don't want

Fatal error: Trait 'Orchestra\Testbench\Concerns\CreatesApplication' not found in /var/source/app/laravel/vendor/nunomaduro/larastan/src/ApplicationResolver.php on line 16

and handle bootstrap by my own.

This PR adds trait_exists to ensure Orchestra\Testbench\Concerns\CreatesApplication exists, which is called in
ApplicationResolver::resolve()
https://github.com/nunomaduro/larastan/blob/master/src/ApplicationResolver.php#L18

[rajyan]

I'll wait for #955

[szepeviktor]

Thank you @rajyan. This is a very good thought.
Could you throw an exception in the else branch explaining that we couldn't find an app nor Testbench?

[szepeviktor]

I'll wait for #955

Resolved in #959
Please proceed.

[rajyan]

@szepeviktor
Thank you for your fix & review.

I added an Exception.
Also, retried #852 , being careful about backward compatibility this time.

[szepeviktor]

Also, retried #852 , being careful about backward compatibility this time.

Please prioritize getcwd() as it was before.

[canvural]

Thank you!