tests.pnpm: rename pnpm_11 test to pnpm_11_v3

Make the fetcher version explicit in the test name in preparation for
adding a v4 test.

Assisted-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pkgs/test/pnpm/default.nix

@@ -2,5 +2,5 @@
 {
   pnpm-empty-lockfile = callPackage ./pnpm-empty-lockfile { };
   pnpm-fixup-state-db = callPackage ./pnpm-fixup-state-db { };
-  pnpm_11 = callPackage ./pnpm_11 { };
+  pnpm_11_v3 = callPackage ./pnpm_11_v3 { };
 }