Commit 1edc1264 replaced Travis CI with GitHub Actions. Remove the
remaining references to Travis CI.

* Bump to PHP 7.2

#3d1210 switched from using phpunit annotations to `expectException ()` for asserting expected exceptions, but the documentation still mentions the annotation.

In https://github.com/simplesamlphp/simplesamlphp/pull/972 we moved the
phpunit config to the root of the project. This was not updated in the
testing documentation.

Update test notes to point to the 4.8 documentation and describe
how to run an old version of phpunit from composer's bin dir.