tests: replace hard-coded /dev/null with variable

- add variable for the null device filename and use that in Perl code.
- initialize this variable with `NUL` on native Windows.
- add `%DEV_NULL` variable and use it in tests.

Fixes `The system cannot find the path specified.` messages seen when
running `runtests.pl` with native Windows Perl.

Also adjust code to not break mcedit syntax highlighting.

Cherry-picked from #14949
Closes #15098

tests/devtest.pl

@@ -110,7 +110,7 @@ sub parseprotocols {
 # Initialize @protocols from the curl binary under test
 #
 sub init_protocols {
-    for (`$CURL -V 2>/dev/null`) {
+    for (`$CURL -V 2>$dev_null`) {
         if(m/^Protocols: (.*)$/) {
             parseprotocols($1);
         }