I'm working on this bug.  I think that what is going on is that the rules to
implement the "make test" targets depend on a lot of unix-like functionality, e.g.:

test_shell_hashed:     darcs
	$(call shell_harness,hashed,\
	 echo ALL --hashed >> .darcs/defaults,cat)

Where shell_harness is:

shell_harness=$(call harness,$1,$2,$3,shell,sh,$(SHELL_HARNESS))

where harness is:

define harness
	@echo Running $4 tests on $1 repositories...
	@rm -rf tests-$4-$1.dir && cp -R tests tests-$4-$1.dir
	@cd tests-$4-$1.dir && rm -rf .darcs; mkdir .darcs;\
	 $2;\
	 if [ -r $(TEST_FILTER_FILE) ];\
	 then grep "\.$5" $(TEST_FILTER_FILE) | grep -v '^#' | $3 | xargs $6; \
	 else ls *.$5 | sort -r | $3 | xargs $6; fi
	@echo Done running tests on $1 repositories...
endef

This is okay in principle -- it just means we require cygwin to run the tests,
which is fine with me -- but I need to figure out what part of it isn't working
on Windows...