Static analysis tools such as PHPStan report errors when dynamic calls are used
for static methods.
```
ERROR Dynamic call to static method PHPUnit\Framework\Assert::assertEquals().
```
According to the source code of PHPUnit
(https://github.com/sebastianbergmann/phpunit/blob/9.5.0/src/Framework/Assert.php)
the function is indeed static.
```php
public static function assertTrue($condition, string $message = ''): void
```
This change updates to PHP Flight test template `register_routes_test.mustache`
to use static calls for PHPUnit assertions.
