Migration tests are very slow in the CI

[Thomas Huth]

 Hi!

Seems like we're getting more timeouts in the CI pipelines since commit 
2649a72555e ("Allow test to run without uffd") enabled the migration tests 
in more scenarios.

For example:

 https://gitlab.com/qemu-project/qemu/-/jobs/2821578332#L49

You can see that the migration-test ran for more than 20 minutes for each 
target (x86 and aarch64)! I think that's way too much by default.

I had a check whether there is one subtest taking a lot of time, but it 
rather seems like each of the migration test is taking 40 to 50 seconds in 
the CI:

 https://gitlab.com/thuth/qemu/-/jobs/2825365836#L44

Given the fact that we're running more than 30 migration tests, this quickly 
sums up to 20 minutes and more.

Could we maybe focus on running only the most important migration tests in 
quick mode, and only run the full suite under an "if (g_test_slow())" statement?

 Thomas