Start a windows/test subdirectory.

This will contain test code and test subprograms that don't belong in
the top-level test directory due to not being cross-platform.

Initial contents are test_screenshot.c, which was already its own
source file in the windows subdir, and test_split_into_argv.c, which
I've sawn off the bottom of windows/utils/split_into_argv.c and moved
into its own source file.

windows/test/test_screenshot.c

@@ -0,0 +1,45 @@
+#include "putty.h"
+
+static NORETURN PRINTF_LIKE(1, 2) void fatal_error(const char *p, ...)
+{
+    va_list ap;
+    fprintf(stderr, "screenshot: ");
+    va_start(ap, p);
+    vfprintf(stderr, p, ap);
+    va_end(ap);
+    fputc('\n', stderr);
+    exit(1);
+}
+
+void out_of_memory(void) { fatal_error("out of memory"); }
+
+int main(int argc, char **argv)
+{
+    const char *outfile = NULL;
+
+    AuxMatchOpt amo = aux_match_opt_init(argc-1, argv+1, 0, fatal_error);
+    while (!aux_match_done(&amo)) {
+        char *val;
+        #define match_opt(...) aux_match_opt( \
+            &amo, NULL, __VA_ARGS__, (const char *)NULL)
+        #define match_optval(...) aux_match_opt( \
+            &amo, &val, __VA_ARGS__, (const char *)NULL)
+
+        if (aux_match_arg(&amo, &val)) {
+            fatal_error("unexpected argument '%s'", val);
+        } else if (match_optval("-o", "--output")) {
+            outfile = val;
+        } else {
+            fatal_error("unrecognised option '%s'\n", amo.argv[amo.index]);
+        }
+    }
+
+    if (!outfile)
+        fatal_error("expected an output file name");
+
+    char *err = save_screenshot(NULL, outfile);
+    if (err)
+        fatal_error("%s", err);
+
+    return 0;
+}