Add tests for creating Mlt instances from a string

[eric-wieser]

This provides coverage for #136

[codecov]

Codecov Report

Merging #337 into master will increase coverage by 0.60%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #337      +/-   ##
==========================================
+ Coverage   74.90%   75.50%   +0.60%     
==========================================
  Files          15       15              
  Lines        4479     4479              
==========================================
+ Hits         3355     3382      +27     
+ Misses       1124     1097      -27     

Impacted Files	Coverage Δ
galgebra/lt.py	57.95% <0.00%> (+4.67%)	⬆️

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