test: remove unmaintainable tests

Most of the tests in this repo have bothered me. They've restricted
progress more than they have helped me make sure I can safely make
changes.

To make sure the action works correctly, I've mostly switched to
github.com/korthout/backport-action-test. That repo verifies the main
functionality, but does not cover all scenarios. I plan to expand these
in the future.

However, edge cases should still be covered by unit-tests. Most of the
existing ones I'm deleting in this commit, because they restrict me. But
it's also noteworthy that they don't really test the edge cases that
they should be testing, nor are they doing this in a good way.

I plan to re-introduce fast running unit tests at a later time. But at
this time, I do not see any reason to keep any of the tests deleted in
this commit.

This commit also updates the README, because the integration tests
(which were aptly called acceptance tests) have been completely removed.
The README still covers how to run tests because we still have some unit
tests that run fast and make sense to have.

README.md

@@ -185,8 +185,6 @@ npm run format && npm run build && npm run package
 
 ## Testing
 
-Tests are located in both src (unit tests) and in [test](test) (integration-style tests).
-
 Run all tests
 
 ```

jest.config.js

@@ -115,7 +115,7 @@ module.exports = {
   // rootDir: "src",
 
   // A list of paths to directories that Jest should use to search for files in
-  roots: ["src", "test"],
+  roots: ["src"],
 
   // Allows you to use a custom runner instead of Jest's default test runner
   // runner: "jest-runner",