Fix illegal reflective access in PointFileReaderWriterTest

[ssoloff]

Overview

Addresses one item in #2796.

The illegal reflective access in PointFileReaderWriterTest was avoided by using an appropriately-configured mock of an interface instead of spying on a concrete class that implements that interface.

Functional Changes

None.

Manual Testing Performed

None.

[codecov-io]

Codecov Report

Merging #4027 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #4027      +/-   ##
============================================
- Coverage     22.98%   22.95%   -0.03%     
+ Complexity     6138     6129       -9     
============================================
  Files           835      835              
  Lines         70913    70913              
  Branches      11466    11466              
============================================
- Hits          16301    16280      -21     
- Misses        52514    52536      +22     
+ Partials       2098     2097       -1

Impacted Files	Coverage Δ	Complexity Δ
...engine/framework/map/download/FileSizeWatcher.java	0% <0%> (-85.72%)	0% <0%> (-5%)
...gy/engine/framework/map/download/DownloadFile.java	36.95% <0%> (-15.22%)	9% <0%> (-2%)
.../games/strategy/engine/ClientFileSystemHelper.java	56.41% <0%> (-2.57%)	10% <0%> (-1%)
.../src/main/java/games/strategy/net/nio/Decoder.java	72.82% <0%> (-1.09%)	24% <0%> (-1%)

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