Integration test is flaky

[rtfb]

It seems to be very rare, but I just stumbled into a case where the integration test broke because the parked hart printed its status faster than the init hart did:

--- testdata/want-output-u64.txt	2022-01-09 14:54:24.705689947 +0000
+++ out/test-output-u64.txt	2022-01-09 14:55:07.326003981 +0000
@@ -1,8 +1,8 @@
+cpu parked: 1
 kinit: cpu 0
 Reading FDT...
 FDT ok
 bootargs: dry-run
 kprintf test several params: foo, 0xF10A, 0
-cpu parked: 1
 KKKK
 qemu-launcher: killing qemu due to timeout
make: *** [Makefile:94: out/test-output-u64.txt] Error 1
Error: Process completed with exit code 2.

This failure makes sense, the harts can execute independently. We need to invent some synchronisation primitive in order to get guaranteed output order.

[rtfb]

The issue with parked harts was fixed in fe3db6a by simply not printing that string. However, the tests are now flaky in another way:

--- testdata/want-smoke-test-output-u64.txt	2022-10-20 17:29:02.503101310 +0000
+++ out/smoke-test-output-u64.txt	2022-10-20 17:31:46.3569[8](https://github.com/rtfb/riscv64-in-qemu/actions/runs/3291666209/jobs/5426107546#step:8:9)66[9](https://github.com/rtfb/riscv64-in-qemu/actions/runs/3291666209/jobs/5426107546#step:8:10)2 +0000
@@ -19,5 +19,6 @@
 1    S      sh
 4    S      hang
 6    R      ps
-
+q
 qemu-launcher: killing qemu due to timeout
+emu-system-riscv64: terminating on signal [15](https://github.com/rtfb/riscv64-in-qemu/actions/runs/3291666209/jobs/5426107546#step:8:16) from pid 44[16](https://github.com/rtfb/riscv64-in-qemu/actions/runs/3291666209/jobs/5426107546#step:8:17) (python3)
make: *** [Makefile:227: out/smoke-test-output-u64.txt] Error 1

This seems to be predominantly happening with the 64 bit version for some reason.