Contextually test either fonts/ttf or fonts/variable

Makefile

@@ -38,7 +38,7 @@ venv-test/touchfile: requirements-test.txt
 	touch venv-test/touchfile
 
 test: venv-test build.stamp
-	. venv-test/bin/activate; mkdir -p out/ out/fontbakery; fontbakery check-googlefonts -l WARN --full-lists --succinct --badges out/badges --html out/fontbakery/fontbakery-report.html --ghmarkdown out/fontbakery/fontbakery-report.md $(shell find fonts/ttf -type f)  || echo '::warning file=sources/config.yaml,title=Fontbakery failures::The fontbakery QA check reported errors in your font. Please check the generated report.'
+	TOCHECK=$$(find fonts/variable -type f 2>/dev/null); if [ -z "$$TOCHECK" ]; then TOCHECK=$$(find fonts/ttf -type f 2>/dev/null); fi ; . venv-test/bin/activate; mkdir -p out/ out/fontbakery; fontbakery check-googlefonts -l WARN --full-lists --succinct --badges out/badges --html out/fontbakery/fontbakery-report.html --ghmarkdown out/fontbakery/fontbakery-report.md $$TOCHECK  || echo '::warning file=sources/config.yaml,title=Fontbakery failures::The fontbakery QA check reported errors in your font. Please check the generated report.'
 
 proof: venv build.stamp
 	. venv/bin/activate; mkdir -p out/ out/proof; diffenator2 proof $(shell find fonts/ttf -type f) -o out/proof