test: allow multiple handlers for the EXIT signal

Bash allows only one handler per signal, so let's overcome this
limitation by having one dedicated EXIT signal which runs all registered
handlers from all over the place.

test/test-functions

@@ -60,6 +60,33 @@ get_bool() {
     fi
 }
 
+# Since in Bash we can have only one handler per signal, let's overcome this
+# limitation by having one global handler for the EXIT signal which executes
+# all registered handlers
+_AT_EXIT_HANDLERS=()
+_at_exit() {
+    set +e
+
+    # Run the EXIT handlers in reverse order
+    for ((i = ${#_AT_EXIT_HANDLERS[@]} - 1; i >= 0; i--)); do
+        ddebug "Running EXIT handler '${_AT_EXIT_HANDLERS[$i]}'"
+        "${_AT_EXIT_HANDLERS[$i]}"
+    done
+}
+
+trap _at_exit EXIT
+
+add_at_exit_handler() {
+    local handler="${1?}"
+
+    if [[ "$(type -t "$handler")" != "function" ]]; then
+        dfatal "'$handler' is not a function"
+        exit 1
+    fi
+
+    _AT_EXIT_HANDLERS+=("$handler")
+}
+
 # Decide if we can (and want to) run qemu with KVM acceleration.
 # Check if nested KVM is explicitly enabled (TEST_NESTED_KVM). If not,
 # check if it's not explicitly disabled (TEST_NO_KVM) and we're not already
@@ -1238,7 +1265,7 @@ cleanup_loopdev() {
     fi
 }
 
-trap cleanup_loopdev EXIT INT QUIT PIPE
+add_at_exit_handler cleanup_loopdev
 
 create_empty_image() {
     if [ -z "${IMAGE_NAME:=}" ]; then