Optionally skip tests relying on 'bitops'

[MichaelChirico]

8 failures if {bitops} is missing:

══ Failed tests ════════════════════════════════════════════════════════════════
── Error (test-namespace.R:22:3): Loaded namespaces have correct version ───────
── Error (test-namespace.R:31:3): Exported objects are visible from global environment ──
── Error (test-namespace.R:55:3): All objects are loaded into namespace environment ──
── Error (test-namespace.R:64:3): All objects are copied to package environment ──
── Error (test-namespace.R:80:3): Unloading and reloading a package works ──────
── Error (test-namespace.R:103:3): Namespace, imports, and package environments have correct hierarchy ──
── Error (test-namespace.R:121:3): unload() removes package environments from search ──
── Error (test-namespace.R:147:3): Environments have the correct attributes ────

Easiest I think to just skip the whole test-namespace.R script if bitops is missing.