BUILD: makefile: implement support for running a command in range

When running "make range", it would be convenient to support running
reg tests or anything else such as "size", "pahole" or even benchmarks.
Such commands are usually specific to the developer's environment, so
let's just pass a generic variable TEST_CMD that is executed as-is if
not empty.

This way it becomes possible to run "make range RANGE=... TEST_CMD=...".

Makefile

@@ -1283,6 +1283,8 @@ unit-tests:
 # options for all commits within RANGE. RANGE may be either a git range
 # such as ref1..ref2 or a single commit, in which case all commits from
 # the master branch to this one will be tested.
+# Will execute TEST_CMD for each commit if defined, and will stop in case of
+# failure.
 
 range:
 	$(Q)[ -d .git/. ] || { echo "## Fatal: \"make $@\" may only be used inside a Git repository."; exit 1; }
@@ -1308,6 +1310,7 @@ range:
 			echo "[ $$index/$$count ]   $$commit #############################"; \
 			git checkout -q $$commit || die 1; \
 			$(MAKE) all || die 1; \
+			[ -z "$(TEST_CMD)" ] || $(TEST_CMD) || die 1; \
 			index=$$((index + 1)); \
 		done; \
 		echo;echo "Done! $${count} commit(s) built successfully for RANGE $${RANGE}" ; \