add RGB -> monochrome read test

[IanButterworth]

Tests the failure in #313 (this should fail on master)

Should be merged with the correct failure before #314

[codecov[bot]]

Codecov Report

Merging #315 (dc902b2) into master (33200c2) will increase coverage by 1.44%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #315      +/-   ##
==========================================
+ Coverage   81.45%   82.90%   +1.44%     
==========================================
  Files          17       17              
  Lines        1181     1503     +322     
==========================================
+ Hits          962     1246     +284     
- Misses        219      257      +38     

Impacted Files	Coverage Δ
src/avio.jl	84.91% <100.00%> (+3.57%)	:arrow_up:
src/util.jl	91.30% <0.00%> (-3.44%)	:arrow_down:
src/testvideos.jl	73.80% <0.00%> (-3.12%)	:arrow_down:
src/avframe_transfer.jl	81.69% <0.00%> (-1.73%)	:arrow_down:
src/avdictionary.jl	8.82% <0.00%> (-0.56%)	:arrow_down:
src/info.jl	100.00% <0.00%> (ø)
src/frame_graph.jl	99.23% <0.00%> (+0.05%)	:arrow_up:
src/encoding.jl	94.73% <0.00%> (+1.53%)	:arrow_up:
src/avptr.jl	69.09% <0.00%> (+2.42%)	:arrow_up:
... and 1 more

---

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 33200c2...dc902b2. Read the comment docs.

[galenlynch]

Thanks for adding this much needed test. If you have the bandwidth, I think it would make sense to test for accuracy, as well as making sure it does not error.

[IanButterworth]

Unfortunately this still doesn't hit the problematic code path. We need a test that hits https://github.com/JuliaIO/VideoIO.jl/blob/33200c28ad823759f7548ca34c5beaf3524b9761/src/avio.jl#L311