maint: avoid :> pattern in tests

This was seen to trigger the EXIT trap on cygwin

* cfg.mk (sc_prohibit_colon_redirection): Disallow all
cases of :> in tests/
* tests/cksum/md5sum-bsd.sh: Adjust to avoid more
stringent syntax check.

cfg.mk

@@ -144,9 +144,10 @@ sc_ensure_gl_diffs_apply_cleanly:
 	done
 
 # Avoid :>file which doesn't propagate errors
+# Also  :>file was seen to trigger EXIT trap on cygwin
 sc_prohibit_colon_redirection:
-	@cd $(srcdir)/tests && GIT_PAGER= git grep -En ': *>.*\|\|'	\
-	  && { echo '$(ME): '"The leading colon in :> will hide errors" >&2; \
+	@cd $(srcdir)/tests && GIT_PAGER= git grep -En ': *>[^>]'	\
+	  && { echo '$(ME): '":> will hide errors or may trigger EXIT" >&2; \
 	       exit 1; }  \
 	  || :
 

tests/cksum/md5sum-bsd.sh

@@ -78,7 +78,7 @@ if echo '' > 'backslash\is\not\dir\sep'; then
   t='	'
   rm check.md5
   for i in 'a\b' 'a\' '\a' "a${nl}b" "a${t}b"; do
-    : > "$i"
+    rm -f "$i" && touch "$i"
     md5sum --tag "$i" >> check.md5 || fail=1
   done
   md5sum --strict -c check.md5 > out || fail=1