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
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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