make NuGet tests more stable

[brettfo]

There have been some recent stability issues with the NuGet tests, specifically the test hanging for several hours. Being perfectly honest, I have no idea what's happening, but I've made the following changes that should make the issues easier to find:

1. C# unit tests are no longer wrapped in a Ruby unit test; they've been migrated directly to ci-test. This makes the test output easier to read.
2. The C# tests are run with a new argument --blame-hang-timeout 5m which means any individual test that is still running after 5 minutes will be killed. I think the longest running C# test is currently ~10 seconds, so the 5 minute limit should be plenty. If this timeout is hit, we'll know exactly what test caused it.
3. The Ruby unit tests are no longer running in parallel (turbo_tests was removed). I suspect this was the real culprit, because in some of the hang cases the C# tests never even got a chance to start.
4. The Ruby unit tests were wrapped in a timeout of 5 minutes. Most Ruby unit tests finish in under a second and the longest is around 5 seconds, so the 5 minute limit should be plenty. If the timeout is hit, we'll know exactly what test caused it.

Downsides to this change. The Ruby unit tests are no longer run in parallel across 4 cores; they're run serially, however this only results in a CI increase of a few seconds; all pure Ruby tests usually run in under 30 seconds anyway.

While repeatedly running the CI, I found some flaky Ruby unit tests in file_updater_spec.rb. I've moved the handling of the temp job file closer to where it's used and I haven't seen the flakiness return.

So far I've run the CI 20 times with the current changes (modulo some minor tweaks) and haven't seen the timeout/hang, where before I would see it ~1/10 times. I will run this several more times to be sure. Apologies to whoever pays the CI bill.

[brettfo]

At this point I've run the CI ~50 times and not seen the original hang, and ~40 times and not seen the flaky Ruby tests. I'm calling this ready.