Move load env var test from TestProgram to XCTest on macOS only

[youming-lin]

For #23

[codecov-io]

Codecov Report

Merging #31 into master will increase coverage by 6.47%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #31      +/-   ##
==========================================
+ Coverage    75.7%   82.18%   +6.47%     
==========================================
  Files           4        4              
  Lines         247      247              
==========================================
+ Hits          187      203      +16     
+ Misses         60       44      -16

Flag	Coverage Δ
#Configuration	82.18% <ø> (+6.47%)	⬆️
#TestProgram	82.18% <ø> (+6.47%)	⬆️

Impacted Files	Coverage Δ
Sources/Configuration/ConfigurationNode.swift	92.03% <0%> (+1.76%)	⬆️
Sources/Configuration/ConfigurationManager.swift	72.65% <0%> (+10.93%)	⬆️

---

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 fee0756...c512b1b. Read the comment docs.