fix: allow default trace function to work

Also add some simple tests for the trace plugin.

tests/unit/40_trace.bats

@@ -0,0 +1,62 @@
+#!/usr/bin/env bats
+
+load test_helper
+
+setup() {
+  global_setup
+  rm -f "$DOKKU_ROOT/.dokkurc/DOKKU_TRACE"
+}
+
+teardown() {
+  rm -f "$DOKKU_ROOT/.dokkurc/DOKKU_TRACE"
+  global_teardown
+}
+
+@test "(trace) trace:on" {
+  run /bin/bash -c "dokku trace:on"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  run /bin/bash -c "test -f $DOKKU_ROOT/.dokkurc/DOKKU_TRACE"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+}
+
+@test "(trace) trace on" {
+  run /bin/bash -c "dokku trace on"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  run /bin/bash -c "test -f $DOKKU_ROOT/.dokkurc/DOKKU_TRACE"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+}
+
+@test "(trace) trace:off, trace off" {
+  touch "$DOKKU_ROOT/.dokkurc/DOKKU_TRACE"
+  run /bin/bash -c "dokku trace:off"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+  run /bin/bash -c "test -f $DOKKU_ROOT/.dokkurc/DOKKU_TRACE"
+  echo "output: $output"
+  echo "status: $status"
+  assert_failure
+
+  touch "$DOKKU_ROOT/.dokkurc/DOKKU_TRACE"
+  run /bin/bash -c "dokku trace off"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+
+
+  run /bin/bash -c "test -f $DOKKU_ROOT/.dokkurc/DOKKU_TRACE"
+  echo "output: $output"
+  echo "status: $status"
+  assert_failure
+}