Acknowledge that the macOS tests are flaky

Restart the tests (at most once) on `unexpected EOF` errors.

This is truly ugly, but might prevent half of the CI runs to fail
because of https://github.com/NixOS/nix/issues/3605

mk/run_test.sh

@@ -14,9 +14,27 @@ if [ -t 1 ]; then
     yellow=""
     normal=""
 fi
+
+run_test () {
     (cd tests && env ${TESTS_ENVIRONMENT} init.sh 2>/dev/null > /dev/null)
     log="$(cd $(dirname $1) && env ${TESTS_ENVIRONMENT} $(basename $1) 2>&1)"
     status=$?
+}
+
+run_test "$1"
+
+# Hack: Retry the test if it fails with “unexpected EOF reading a line” as these
+# appear randomly without anyone knowing why.
+# See https://github.com/NixOS/nix/issues/3605 for more info
+if [[ $status -ne 0 && $status -ne 99 && \
+    "$(uname)" == "Darwin" && \
+    "$log" =~ "unexpected EOF reading a line"
+]]; then
+    echo "$post_run_msg [${yellow}FAIL$normal] (possibly flaky, so will be retried)"
+    echo "$log" | sed 's/^/    /'
+    run_test "$1"
+fi
+
 if [ $status -eq 0 ]; then
   echo "$post_run_msg [${green}PASS$normal]"
 elif [ $status -eq 99 ]; then