Fix PositionStream to report correct position; add tests for reading files

[ChrisJefferson]

This fixes a bug in PositionStream, and also adds a big general test for reading of files, and positions in streams.

[codecov]

Codecov Report

Merging #2398 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2398      +/-   ##
==========================================
+ Coverage   73.82%   73.83%   +<.01%     
==========================================
  Files         484      484              
  Lines      245793   245799       +6     
==========================================
+ Hits       181463   181484      +21     
+ Misses      64330    64315      -15

Impacted Files	Coverage Δ
src/streams.c	73.18% <100%> (+0.46%)	⬆️
src/hpc/threadapi.c	43.3% <0%> (-0.11%)	⬇️
src/gasman.c	85.66% <0%> (+0.18%)	⬆️
src/sysfiles.c	37.71% <0%> (+0.35%)	⬆️
hpcgap/lib/hpc/stdtasks.g	64.7% <0%> (+0.51%)	⬆️
lib/streams.gi	31.92% <0%> (+1.26%)	⬆️

[fingolfin]

Looks good to me. I have only minor nitpicks, but it's also fine to just ignore those.

[fingolfin]

In case you end up making other changes: perhaps remove second empty line? or even both empty lines, and then for symmetry change ret = ... to Int ret = ..., and remove the declaration of ret above?

[fingolfin]

so... testName is only used for printing in error messages, presumably to identify the failing tests case? openFunc opens a new stream, lines contains the expected content of that stream, and hasPosition indicates whether PositionStream is supported. Correct?

Perhaps add this into a comment, and/or rename lines to expectedLines or expected; and hasPosition to supportsPositionStream?

[fingolfin]

Clarification for release notes: the bug in PositionStream was that it did not take buffering into account, and thus could return incorrect values.