Added letter library tests #1057

Job	Run time
Set CI/CD metadata	5s
Commit stage / Check file format	6s
Commit stage / Check English usage	7s
Commit stage / Count lines of code	16s
Commit stage / Scan dependencies	23s
Commit stage / Check Markdown format	10s
Commit stage / Scan secrets	9s
Test stage	-1s
	1m 15s

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