fix(ci): update @denops/test to v3.1.0 for @denops/core v8 compatibility

The CI failures were caused by @denops/test v3.0.4 being hardcoded to use
@denops/core v7, while the project uses v8. This version mismatch caused
fs.close() errors in the Linux CI environment.

Updating to @denops/test v3.1.0 which supports @denops/core v8.

Author: lambdalisue

deno.jsonc

@@ -47,8 +47,8 @@
   "imports": {
     "@core/asyncutil": "jsr:@core/asyncutil@^1.2.0",
     "@core/unknownutil": "jsr:@core/unknownutil@^4.3.0",
-    "@denops/core": "jsr:@denops/core@^7.0.0",
-    "@denops/test": "jsr:@denops/test@^3.0.4",
+    "@denops/core": "jsr:@denops/core@^8.0.0",
+    "@denops/test": "jsr:@denops/test@^3.1.0",
     "@lambdalisue/errorutil": "jsr:@lambdalisue/errorutil@^1.1.1",
     "@lambdalisue/itertools": "jsr:@lambdalisue/itertools@^1.1.2",
     "@lambdalisue/unreachable": "jsr:@lambdalisue/unreachable@^1.0.1",