Add test for buffering behavior of iostreams

[Flamefire]

Closes #186

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.47%. Comparing base (bac0f20) to head (5fffda9).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #187      +/-   ##
===========================================
+ Coverage    99.45%   99.47%   +0.02%     
===========================================
  Files           34       34              
  Lines         3319     3460     +141     
===========================================
+ Hits          3301     3442     +141     
  Misses          18       18              

Files with missing lines	Coverage Δ
src/iostream.cpp	100.00% <100.00%> (ø)
test/test_iostream.cpp	100.00% <100.00%> (ø)

... and 20 files with indirect coverage changes