run-tests: Use mingw32-make in busybox

This is the make tool we'd expect to use in such a setup.

run-tests.sh

@@ -33,6 +33,13 @@ MAKE=make
 if command -v gmake >/dev/null; then
     MAKE=gmake
 fi
+if [ "$(uname)" = "Windows_NT" ]; then
+    # In busybox, prefer our mingw32-make. The plain "make" may be busybox's
+    # primitive make (although in our builds of busybox, we disable that),
+    # and "gmake" can be a different build from e.g. Strawberry Perl further
+    # back in PATH.
+    MAKE=mingw32-make
+fi
 
 case $(uname) in
 Darwin)