Allow custom nosetests parameters

The NOSETESTS variable can be used to support customizing the nosetests
parameters.  This allows providing custom nose parameters such as
--with-coverage.

Makefile

@@ -55,7 +55,7 @@ ifeq ($(OFFICIAL),)
 endif
 RPMNVR = "$(NAME)-$(VERSION)-$(RPMRELEASE)$(RPMDIST)"
 
-NOSETESTS := nosetests
+NOSETESTS ?= nosetests
 
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