[TST] Additional smoke tests for utils.py

[monicayao]

Closes #335 .

Changes proposed in this pull request:

• Added smoke tests to the functions in utils.py
• Many functions, when they had only one output, were simply used in the assertion line (this may be edited, please give me your inputs!)

[codecov]

Codecov Report

Merging #377 into master will increase coverage by 0.74%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #377      +/-   ##
==========================================
+ Coverage   79.74%   80.48%   +0.74%     
==========================================
  Files          40       40              
  Lines        2236     2255      +19     
==========================================
+ Hits         1783     1815      +32     
+ Misses        453      440      -13

Impacted Files	Coverage Δ
tedana/tests/test_utils.py	100% <100%> (ø)	⬆️
tedana/tests/test_decay.py	100% <0%> (ø)	⬆️
tedana/utils.py	96.19% <0%> (+12.38%)	⬆️

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[emdupre]

Thanks so much, @monicayao !