[1.15] Update tests to v1.15

[yurabakhtin]

Issue: https://github.com/humhub/humhub-internal/issues/191

Need this fix because some methods were modified to static here humhub/humhub@4a29d7a for v1.15.

[luke-]

@yurabakhtin Can we restore the master test file?

[yurabakhtin]

@yurabakhtin Can we restore the master test file?

@luke- Yes, we can, but I am not sure we should do this because it was not done by me here humhub/humhub#6480.
As I understand PHP allows to call static methods as $this->method(), so we should not touch modules where only the method calls are used, but this module "Tasks" redefines the parent method assertHasNoNotification, and I have compared the tasks child method with core parent method and find only one difference:

• core: 'source_class' => PolymorphicRelation::getObjectModel($source),
• module: 'source_class' => $source->className(),

I think results are same, so I have just removed the methods from tasks module side, also I have reverted min version to 1.14, please merge this PR because it works well on previous min version and on all others as you can see all commits have all tests as green.

[luke-]

@yurabakhtin Thanks, the solution looks good to me.