Simplify BYPRODUCTS in "make tree"

The inner structure of mktree.output/ should be treated as an
implementation detail of the mktree backend in use.  All we care for
here is that it's purged on a "make clean".

No functional change since only mktree.fedora uses this directory
currently and it already recreates the other contents (inst/) on each
rebuild anyway.

tests/CMakeLists.txt

@@ -137,7 +137,7 @@ add_custom_command(OUTPUT rpmtests
 
 add_custom_target(tree
 	COMMAND ./mktree build
-	BYPRODUCTS mktree.output/base
+	BYPRODUCTS mktree.output
 	DEPENDS rpmtests
 	DEPENDS ${TESTPROGS}
 	DEPENDS ${TOP_TARGETS}