BUILD: add REGTESTS_KEEP_LOGS variable to preserve regression test logs

Add support for a new REGTESTS_KEEP_LOGS environment variable that can be
passed to the regression test script to keep all test logs after completion.

This is useful for debugging and analyzing test failures without having to
modify the test runner or reconfigure the build system.

Usage:
$ REGTESTS_KEEP_LOGS=1 make reg-tests

Author: chipitsine

Makefile

@@ -1265,7 +1265,7 @@ endif
 
 # Target to run the regression testing script files.
 reg-tests:
-	$(Q)$(REG_TEST_SCRIPT) --type "$(REGTESTS_TYPES)" $(REGTEST_ARGS) $(REG_TEST_FILES)
+	$(Q)$(REG_TEST_SCRIPT) --type "$(REGTESTS_TYPES)" $(if $(REGTESTS_KEEP_LOGS),--keep-logs) $(REGTEST_ARGS) $(REG_TEST_FILES)
 .PHONY: $(REGTEST_ARGS)
 
 reg-tests-help:
@@ -1288,6 +1288,9 @@ reg-tests-help:
 	@echo "To run tests with specific types:"
 	@echo "    $$ REGTESTS_TYPES=slow,default make reg-tests"
 	@echo
+	@echo "To keep all reg-tests logs:"
+	@echo "    $$ REGTESTS_KEEP_LOGS=1 make reg-tests"
+	@echo
 	@echo "with 'default,bug,devel,slow' as default value for REGTESTS_TYPES variable."
 	@echo
 	@echo "About the reg test types:"