Define constant for short string length #95

Job	Run time
busted (luajit-openresty, ubuntu-latest)	24s
busted (luajit-openresty, windows-latest)	1m 34s
busted (luajit-openresty, macos-latest)	53s
busted (luajit-2.1.0-beta3, ubuntu-latest)	24s
busted (luajit-2.1.0-beta3, windows-latest)	2m 5s
busted (luajit-2.1.0-beta3, macos-latest)	1m 18s
busted (luajit-git, ubuntu-latest)	24s
busted (luajit-git, windows-latest)	1m 30s
busted (luajit-git, macos-latest)	1m 1s
busted (5.4.6, ubuntu-latest)	27s
busted (5.4.6, windows-latest)	1m 43s
busted (5.4.6, macos-latest)	48s
busted (5.1.5, ubuntu-latest)	26s
busted (5.1.5, windows-latest)	1m 32s
busted (5.1.5, macos-latest)	1m 11s
busted (5.3.6, ubuntu-latest)	31s
busted (5.3.6, windows-latest)	1m 51s
busted (5.3.6, macos-latest)	47s
busted (luajit-git, ubuntu-latest, rust)	42s
finish	2s
publish	0s
	19m 33s

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