vcsaddons tests sometimes failed in circleci

[muryanto1]

Example of failure:
https://circleci.com/gh/CDAT/vcsaddons/1546

b'Test file  : tests_png/test_vcsaddons_polar_sub_mag_ticks.png'
b'Source file: tests/baselines/test_vcsaddons_polar_sub_mag_ticks.png'
Executing nosetests -s /Users/distiller/project/tests/test_vcsaddons_polar_basic.py in /Users/distiller/project
b'2020-07-16 18:10:45.114 (   3.661s) [            47A8]       vtkPNGReader.cxx:144    ERR| (nullptr): PNGReader error reading file. Premature EOF while reading header.'
b'2020-07-16 18:10:45.114 (   3.661s) [            47A8]       vtkPNGReader.cxx:269    ERR| vtkPNGReader (0x7f894b5da8d0): Invalid file header: not a PNG file'
b'2020-07-16 18:10:45.114 (   3.661s) [            47A8]       vtkPNGReader.cxx:144    ERR| (nullptr): PNGReader error reading file. Premature EOF while reading header.'
b'2020-07-16 18:10:45.114 (   3.661s) [            47A8]       vtkPNGReader.cxx:393    ERR| vtkPNGReader (0x7f894b5da8d0): Invalid file header: not a PNG file'
b'2020-07-16 18:10:45.114 (   3.661s) [            47A8]vtkImageExtractComponen:230    ERR| vtkImageExtractComponents (0x7f894b5d3110): Execute: Component 1 is not in input.'
b'2020-07-16 18:10:45.127 (   3.674s) [            47A8] vtkImageDifference.cxx:569    ERR| vtkImageDifference (0x7f894ba35260): ExecuteInformation: Input are not the same size.'
b' Input1 is: 0,0,0,0,0,0'
b' Input2 is: 0,1089,0,1199,0,0'
b'2020-07-16 18:10:45.127 (   3.674s) [            47A8] vtkImageDifference.cxx:534    ERR| vtkImageDifference (0x7f894ba35260): RequestData: All ScalarTypes must be unsigned char'
b'2020-07-16 18:10:45.127 (   3.674s) [            47A8]       vtkExecutive.cxx:753    ERR| vtkCompositeDataPipeline (0x7f894b9eca80): Algorithm vtkImageDifference(0x7f894ba35260) returned failure for request: vtkInformation (0x7f894ba6beb0)'
b'  Debug: Off'
b'  Modified Time: 98716'
b'  Reference Count: 1'
b'  Registered Events: (none)'
b'  Request: REQUEST_DATA'
b'  FROM_OUTPUT_PORT: 0'
b'  ALGORITHM_AFTER_FORWARD: 1'
b'  FORWARD_DIRECTION: 0'
b'2020-07-16 18:10:45.127 (   3.675s) [            47A8]       vtkPNGWriter.cxx:212   WARN| vtkPNGWriter (0x7f894b0d0ed0): PNGWriter only supports unsigned char and unsigned short inputs'
b'F'
b'======================================================================'
b'FAIL: test00 (test_EzTemplate_0.VCSAddonTest)'

and the failing test case is random but with similar failure.