Automated testing is not magic. it only tests things you make it test.
And since kernel is basically involved into everything that computer does, you would have to test everything, to find every error. Linux kernel is also extremely customizable, so that it can run from watches, to mars rowers, to HPC and cloud.
Linux kernels are continuously heavily tested on various different hardware architectures, by various organizations (RH, IBM, Oracle, Google).
So there are automated tests done. It's just seems nobody tested this.
Of course. Thanks for stating a lot of mostly obvious things.
Assuming there is a test suite (I really don't know enough about Linux kernel testing, sorry), it is a matter of testing that on a representative set of configurations. Apparently no-one thought of having one with a swap file instead of a swap partition. That's okay, it's an understandable oversight, but one that clearly should be remedied.
