fix long duration of write_to_full test case and max_io test case

[yamingk]

1. change max_io size to 60MB which will fit in with existing portion nblks with 4GB min chunk size;
2. change write_to_full case to allow use file for testing to complete sooner. The original purpose of this test case is to see what meta svc will behave after writing laste byte of its vdev. If with real drives, it will take around 3 days to complete this single test case. We can choose to remove --use_file if we do really want to test that long with meta svc some day...

Testing:

1. both max io test case and write to full test case passed by manual run on real drives.
2. a new round of meta svc long run is kicked off and waiting to complete (will merge after a full round passed).

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.89%. Comparing base (1a0cef8) to head (6fc2f85).
Report is 57 commits behind head on master.

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Additional details and impacted files

@@             Coverage Diff             @@
##           master     #533       +/-   ##
===========================================
+ Coverage   56.51%   67.89%   +11.38%     
===========================================
  Files         108      109        +1     
  Lines       10300    10435      +135     
  Branches     1402     1400        -2     
===========================================
+ Hits         5821     7085     +1264     
+ Misses       3894     2681     -1213     
- Partials      585      669       +84     

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[xiaoxichen]

lgtm

[JacksonYao287]

LG