Tests

Add `dummy_syntaxes` input to create dummy syntaxes #97

Job	Run time
Without default packages, old build	18s
Without default packages	3s
Dummy Syntax	3s
With default packages, latest stable	6s
Without default packages, latest build, ubuntu-20.04	6s
ST3 build	21s
Error on purpose	7s
Third-party	3s
	1m 7s

Provide feedback