Reduce test running time

[Yue-Zhengyuan]

Currently a run of all tests on GitHub need 2 to 3 hours. As @lkdvos once mentioned (#291 (comment)), we need to try coming up with more elementary tests, instead of using full runs of physical examples, to reduce the running time of tests, and catch errors more quickly and precisely.

[pbrehmer]

I think for most parts of the test suite, we're actually completely fine. Many things take just about 5-10 minutes and even the extensive CTMRG tests (which could be made more elementary) take only around 20-30 minutes. The real trouble comes from anything that includes AD gradients of CTMRG, i.e. the example and gradient test sets.

For those tests, the actual time spent in running the code is not that bad I think - most of the time is spent in precompiling Zygote derivatives. Since we test many different cases in both the examples and gradient cases (different symmetries, fermions, etc.), the backwards pass needs to be precompiled many times as well. I don't think more elementary tests would get rid of this compilation time. So if we really want to cut down on the test suite runtime, we need to do something about precompilation.

Actually, I remember that up to a certain point, the precompilation wouldn't take so terribly long and I'm not sure what happened in the meantime such that it changed. But after some updates, the example and gradient tests suddenly took much longer.