test: improve util-format code coverage

Add test case for calling `util.format()` with a BigInt and a separator.

Refs: https://coverage.nodejs.org/coverage-986cf3b986c6c3e1/lib/internal/util/inspect.js.html#L2042

PR-URL: #41572
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Mestery <mestery@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Mohammed Keyvanzadeh <mohammadkeyvanzade94@gmail.com>
Reviewed-By: Zeyu Yang <himself65@outlook.com>

test/parallel/test-util-format.js

@@ -143,6 +143,13 @@ assert.strictEqual(
   '1180591620717411303424n 12345678901234567890123n'
 );
 
+assert.strictEqual(
+  util.formatWithOptions(
+    { numericSeparator: true },
+    '%i %d', 1180591620717411303424n, 12345678901234567890123n),
+  '1_180_591_620_717_411_303_424n 12_345_678_901_234_567_890_123n'
+);
+
 // Float format specifier
 assert.strictEqual(util.format('%f'), '%f');
 assert.strictEqual(util.format('%f', 42.0), '42');