w: don't fail when tty device is missing

[Franklin-Qi]

• Treat missing or unreadable /dev/<tty> as zero idle time instead of exiting with an error
• Add unit test for missing tty handling

Closes: #718

[sylvestre]

can we also get a test in https://github.com/uutils/procps/blob/main/tests/by-util/test_w.rs ? thanks

[codecov]

Codecov Report

❌ Patch coverage is 0% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 19.18%. Comparing base (9f4a1ea) to head (012b57f).
⚠️ Report is 60 commits behind head on main.

Files with missing lines	Patch %	Lines
src/uu/w/src/w.rs	0.00%	10 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #719 +/- ##
==========================================
- Coverage 19.19% 19.18% -0.01% 
==========================================
Files 68 68 Lines 10158 10162 +4 Branches 546 549 +3 ==========================================
Hits 1950 1950 - Misses 8208 8212 +4 

Flag	Coverage Δ
macos_latest	6.74% <ø> (ø)
ubuntu_latest	19.54% <0.00%> (-0.01%)	⬇️
windows_latest	0.02% <ø> (ø)

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[Franklin-Qi]

can we also get a test in https://github.com/uutils/procps/blob/main/tests/by-util/test_w.rs ? thanks

@sylvestre Thank you for the review; I will add test cases.