Fix failing nanosec truncation check on mac OS

[k-yomo]

Fixes #1421

Nanosecond level precision is already truncated on some platforms including mac OS.
This PR fixes the failing test by comparing with microsecond, and I believe it's a little more valid test than asserting the inequality.

Related issue found on GitHub: edgedb/edgedb-go#209

[codecov-commenter]

Codecov Report

Merging #1423 (b5a9843) into main (af84e74) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1423      +/-   ##
==========================================
- Coverage   94.22%   94.22%   -0.01%     
==========================================
  Files         236      236              
  Lines       43775    43775              
==========================================
- Hits        41248    41245       -3     
- Misses       2527     2530       +3     

Impacted Files	Coverage Δ
src/lib.rs	96.48% <100.00%> (ø)
src/directory/directory.rs	90.14% <0.00%> (-1.41%)	⬇️
src/fastfield/reader.rs	89.07% <0.00%> (-0.85%)	⬇️
src/postings/stacker/expull.rs	98.57% <0.00%> (-0.48%)	⬇️

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[lavrd]

Thanks!