async-stream: lots more tests

[graebm]

Expand to have more than just 3 tests to stress aws_async_input_stream

• when Content-Length is known:
  • test single-part (goes through "default" flow, all other tests go through MPU flow)
  • vanilla multi-part test
  • test when aws_async_input_stream_read() actually completes synchronously
  • test when async read() doesn't fill the buffer
    • threw in test to check this for synchronous reads too
• when Content-Length is unknown (always goes through MPU flow):
  • test length < part size
  • test length == part size
  • test length == part size BUT we don't realize it's at EOF util we try to read the 2nd part
  • test length > part size

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Merging #311 (777ce92) into main (2417288) will increase coverage by 0.06%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #311      +/-   ##
==========================================
+ Coverage   88.95%   89.01%   +0.06%     
==========================================
  Files          17       17              
  Lines        4936     4936              
==========================================
+ Hits         4391     4394       +3     
+ Misses        545      542       -3     

see 2 files with indirect coverage changes