tests: Properly clean up .bak file from tests

One of our testcases creates a .bak file from invoking sed.  Fix that to be
removed by make clean, and also ignore it in git to avoid clutter.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

.gitignore

@@ -4,6 +4,7 @@
 *.patch
 *.so
 *~
+*.bak
 *.tab.[ch]
 lex.yy.c
 *.lex.c

tests/Makefile.tests

@@ -49,7 +49,7 @@ TESTS_DEPFILES = $(TESTS:%=%.d) \
 	$(addprefix $(TESTS_PREFIX),testutils.d trees.d dumptrees.d)
 
 TESTS_CLEANFILES_L = $(STD_CLEANFILES) \
-	*.dtb *.test.dts *.test.dt.yaml *.dtsv1 tmp.* \
+	*.dtb *.test.dts *.test.dt.yaml *.dtsv1 tmp.* *.bak \
 	dumptrees
 TESTS_CLEANFILES = $(TESTS) $(TESTS_CLEANFILES_L:%=$(TESTS_PREFIX)%)
 TESTS_CLEANDIRS_L = fs