Refactor tests

[jasonrhansen]

No description provided.

[jonhoo]

LGTM subject to those two minor comments.

[codecov-io]

Codecov Report

Merging #70 into master will increase coverage by 0.03%.
The diff coverage is 97.33%.

@@            Coverage Diff            @@
##           master     #70      +/-   ##
=========================================
+ Coverage   84.77%   84.8%   +0.03%     
=========================================
  Files          13      13              
  Lines        1392    1408      +16     
=========================================
+ Hits         1180    1194      +14     
- Misses        212     214       +2

Impacted Files	Coverage Δ
src/bin/flamegraph.rs	0% <0%> (ø)	⬆️
src/flamegraph/mod.rs	97.88% <100%> (+0.03%)	⬆️
src/flamegraph/svg.rs	98.41% <100%> (+0.01%)	⬆️
tests/flamegraph.rs	94.54% <100%> (+0.17%)	⬆️
tests/collapse-perf.rs	95.94% <100%> (+0.23%)	⬆️

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