Add a comment to 90-test_external.t to explain why we need filter_run

Also rename executable to cmd...otherwise it breaks!
Reviewed-by: NRichard Levitte <levitte@openssl.org>

test/recipes/90-test_external.t

@@ -20,14 +20,17 @@ if (!$ENV{BORING_RUNNER_DIR}) {
 plan tests => 1;
 
 indir $ENV{BORING_RUNNER_DIR} => sub {
-    ok(filter_run(executable(["go", "test", "-shim-path",
-                              srctop_file("test", "ossl_shim", "ossl_shim"),
-                              "-shim-config",
-                              srctop_file("test", "ossl_shim",
-                                          "ossl_config.json"),
-                              "-pipe"])),
+    ok(filter_run(cmd(["go", "test", "-shim-path",
+                      srctop_file("test", "ossl_shim", "ossl_shim"),
+                      "-shim-config",
+                      srctop_file("test", "ossl_shim", "ossl_config.json"),
+                      "-pipe"])),
+        "running external tests");
 }, create => 0, cleanup => 0;
 
+# Filter the output so that the "ok" printed by go test doesn't confuse
+# Test::More. Without that it thinks there has been one more test run than was
+# planned
 sub filter_run {
     my $cmd = cmdstr(shift);
     open(PIPE, "-|", $cmd);