test: add coverage for util.inspect()

Coverage stats indicate that there is no coverage for util.inspect()
with a negative number and a numeric separator. Add a test case.

Refs: https://coverage.nodejs.org/coverage-df507758e6c35534/lib/internal/util/inspect.js.html#L1463

Author: Trott

test/parallel/test-util-inspect.js

@@ -3231,4 +3231,9 @@ assert.strictEqual(
     util.inspect(123456789.12345678, { numericSeparator: true }),
     '123_456_789.123_456_78'
   );
+
+  assert.strictEqual(
+    util.inspect(-123456789.12345678, { numericSeparator: true }),
+    '-123_456_789.123_456_78'
+  );
 }