test-boot: run in parallel

--quit-after-boot: fix for gem5, update path to gem5.sh

Improve the printing of results and errors:

- remove newlines from IDs at the end for ./test-boot
- remove newlines from progress for __call__ commands and don't print executed commands at all,
  otherwise there are too many lines per test and it is hard to tell what is going on
- print backtraces for any exception in the threads (bugs while developing this code)

Tests across different archs and emulators are still not running in parallel,
which is a huge loss. TODO.

thread_pool: introduce with API. This was motivate by test-boot, I've had enough
of doing separate error handling for each loop type! Greatly dries up the code, awesome.

common: make --all-emulators work properly with native hopefully for the last time,
./test-baremetal was still failing.

gem5: don't pass --command-line for baremetal. Maybe later we can use it to actually
pass command line arguments to main()? To be seen.

build-test-boot

@@ -3,7 +3,7 @@
 # what to build depends on --size, which ./build does not support right now.
 # The best way to solve this is to move the dependency checking into the run
 # scripts, which will take a while to refactor.
-set -eu
+set -eux
 test_size=1
 while [ $# -gt 0 ]; do
   case "$1" in