test(url): make url_exists tests deterministic and offline

Fix flaky network-dependent URL tests in the shell test suite.

Root cause:
- tests used external hosts (facebook.com, google.com)
- DNS/network availability varies by environment, causing false negatives

Changes:
- app/test-url-exists-ok now checks local file URLs:
  file:///etc/hosts and file:///etc/passwd
- app/test-url-exists-no now checks one failing and one passing local URL:
  file:///definitely-not-here and file:///etc/hosts
- update SPDX copyright year to 2026 in both files

Validation:
- shellcheck -s sh app/test-url-exists-ok app/test-url-exists-no
- redo test (passes across available shells)

Author: rdavid

app/test-url-exists-no

@@ -1,9 +1,9 @@
 #!/bin/sh
 # vi:et lbr noet sw=2 ts=2 tw=79 wrap
-# SPDX-FileCopyrightText: 2022-2025 David Rabkin
+# SPDX-FileCopyrightText: 2022-2026 David Rabkin
 # SPDX-License-Identifier: 0BSD
 # shellcheck disable=SC1090,SC1091 # File not following.
 . "$(CDPATH='' cd -- "$(dirname -- "$0")" && pwd -P)"/../lib/base.sh
 url_exists \
-	doesnotexist.c0m \
-	google.com
+	file:///definitely-not-here \
+	file:///etc/hosts

app/test-url-exists-ok

@@ -1,9 +1,9 @@
 #!/bin/sh
 # vi:et lbr noet sw=2 ts=2 tw=79 wrap
-# SPDX-FileCopyrightText: 2022-2025 David Rabkin
+# SPDX-FileCopyrightText: 2022-2026 David Rabkin
 # SPDX-License-Identifier: 0BSD
 # shellcheck disable=SC1090,SC1091 # File not following.
 . "$(CDPATH='' cd -- "$(dirname -- "$0")" && pwd -P)"/../lib/base.sh
 url_exists \
-	facebook.com \
-	google.com
+	file:///etc/hosts \
+	file:///etc/passwd