defensive testing: locale command may not be available

[bastistician]

On my Alpine Linux check system, I see

  ══ Failed tests ════════════════════════════════════════════════════════════════
  ── Error ('test-gt_object.R:188:3'): A gt table can use UTF-8 chars in any system locale ──
  <cmdError/error/condition>
  Error in `system("locale -a", intern = TRUE)`: error in running command
  Backtrace:
      ▆
   1. └─base::system("locale -a", intern = TRUE) at test-gt_object.R:188:2
  
  [ FAIL 1 | WARN 0 | SKIP 0 | PASS 799 ]
  Error: Test failures

This PR simply skips this test if locale -a fails to run (it is already skipped on Windows as well).
With this change, gt 0.9.0 checks fine on my system.

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[rich-iannone]

lgtm!

[rich-iannone]

And sorry this took so long to review. Will merge now though!