[tests] Fixed flaky selenium test: test_autocomplete_shelf_filter

[youhaveme9]

Checklist

• I have read the OpenWISP Contributing Guidelines.
• I have manually tested the changes proposed in this pull request.
• I have written new test cases for new code and/or updated existing tests for changes to existing code.
• I have updated the documentation.

Reference to Existing Issue

Closes #400

[coveralls]

coverage: 96.18%. remained the same
when pulling 829f73b on youhaveme9:issues/400-flaky-test-autocomplete-shelf-filter
into 1011f52 on openwisp:master.

[nemesifier]

Thanks, LGTM!