Specify --assert=plain/rewrite for tests.

[joseph-roitman]

Test with both --assert=rewrite and --assert=plain.
This should help catch bugs such as #49.

[codecov]

Codecov Report

Merging #51 (296bdaf) into master (c1762ed) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #51      +/-   ##
==========================================
+ Coverage   99.56%   99.58%   +0.01%     
==========================================
  Files           8        8              
  Lines         461      482      +21     
  Branches       54       58       +4     
==========================================
+ Hits          459      480      +21     
  Misses          2        2              

Impacted Files	Coverage Δ
tests/test_assert_match.py	100.00% <100.00%> (ø)
tests/test_assert_match_dir.py	100.00% <100.00%> (ø)
tests/test_misc.py	100.00% <100.00%> (ø)
tests/utils.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c1762ed...296bdaf. Read the comment docs.