ci: test on armhf and arm64

This MR adds build+unit/config test coverage on 32b and 64b ARM.

The runners are Debian 10 LXC containers running on Turris Omnia (armhf) and Turris Mox (arm64).

Consider these two tests experimental and feel free to temporarily disable/ignore them in case something happens with the runners. Also, I've seen a few weird errors with config.doh2 on arm64, although I didn't reliably reproduce them - I'm not sure if those are real issues or not.

.gitlab-ci.yml

@@ -101,7 +101,7 @@ build:armhf:
     - meson build_ci_armhf --default-library=static --prefix=$PREFIX -Dwerror=true -Dextra_tests=enabled
     - ninja -C build_ci_armhf
     - ninja -C build_ci_armhf install >/dev/null
-    - ${MESON_TEST} --suite unit --suite config --no-suite snowflake --no-suite y2k38
+    - MESON_TESTTHREADS=1 ${MESON_TEST} --suite unit --suite config --no-suite snowflake --no-suite y2k38
 
 build:arm64:
   <<: *build
@@ -112,7 +112,7 @@ build:arm64:
     - meson build_ci_arm64 --default-library=static --prefix=$PREFIX -Dwerror=true -Dextra_tests=enabled
     - ninja -C build_ci_arm64
     - ninja -C build_ci_arm64 install >/dev/null
-    - ${MESON_TEST} --suite unit --suite config --no-suite snowflake
+    - MESON_TESTTHREADS=1 ${MESON_TEST} --suite unit --suite config --no-suite snowflake
 
 build-asan:
   <<: *build
@@ -123,7 +123,7 @@ build-asan:
     - ninja -C build_ci_asan install >/dev/null
       # TODO _leaks: not sure what exactly is wrong in leak detection on config tests
       # TODO skip_asan: all three of these disappear locally when using gcc 9.1 (except some leaks)
-    - ASAN_OPTIONS=detect_leaks=0 ${MESON_TEST} --suite unit --suite config --suite dnstap --no-suite skip_asan --no-suite snowflake
+    - MESON_TESTTHREADS=1 ASAN_OPTIONS=detect_leaks=0 ${MESON_TEST} --suite unit --suite config --suite dnstap --no-suite skip_asan --no-suite snowflake
 
 build:macOS:
   <<: *nodep