Do not use predefined packages for testing

[koehnlein]

• Please check if the PR fulfills these requirements
• [ ] Tests for the changes have been added (for bug fixes / features)

The complete PR is about testing 😄 But I'm pretty sure, the tests will fail when merged ...

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Fix tests

• What is the current behavior? (You can also link to an open issue here)

Because of the existing composer.lock file and the definition of PHP 7.4 in the composer.json, the tests in the pipeline are always based on the predefined packages. This results in surf not being tested with newer packages that would be used if someone installs surf into their environment via composer.

• What is the new behavior (if this is a feature change)?

Delete the composer.lock file, which allows the pipeline to install latest packages.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

It does not affect any user's installation, but I'm pretty sure, the tests will fail, because there are some problems with newer einvironments (see #760)

• Other information:

[sabbelasichon]

@koehnlein Thanks for your investigation. I have created a separated complete PR #770 based on your changes. I think we should give it a try.