many-seeds tests have become a lot faster thanks to multithreading

Author: RalfJung

ci/ci.sh

@@ -142,12 +142,10 @@ case $HOST_TARGET in
     # Host
     GC_STRESS=1 MIR_OPT=1 MANY_SEEDS=64 TEST_BENCH=1 CARGO_MIRI_ENV=1 run_tests
     # Extra tier 1
-    # With reduced many-seed count to avoid spending too much time on that.
-    # (All OSes and ABIs are run with 64 seeds at least once though via the macOS runner.)
-    MANY_SEEDS=16 TEST_TARGET=i686-unknown-linux-gnu run_tests
-    MANY_SEEDS=16 TEST_TARGET=aarch64-unknown-linux-gnu run_tests
-    MANY_SEEDS=16 TEST_TARGET=x86_64-apple-darwin run_tests
-    MANY_SEEDS=16 TEST_TARGET=x86_64-pc-windows-gnu run_tests
+    MANY_SEEDS=64 TEST_TARGET=i686-unknown-linux-gnu run_tests
+    MANY_SEEDS=64 TEST_TARGET=aarch64-unknown-linux-gnu run_tests
+    MANY_SEEDS=64 TEST_TARGET=x86_64-apple-darwin run_tests
+    MANY_SEEDS=64 TEST_TARGET=x86_64-pc-windows-gnu run_tests
     ;;
   aarch64-apple-darwin)
     # Host