Fix error in UnitTestError

Locally, I am observing the following test failure:

    Start 117: UnitTestError

117: Test command: /Users/4nt/vtk-m/build/bin/UnitTests_vtkm_cont_testing "UnitTestError" "-v" "INFO" "--data-dir=/Users/4nt/vtk-m/data/data" "--baseline-dir=/Users/4nt/vtk-m/data/baseline" "--write-dir=/Users/4nt/vtk-m/build"
117: Test timeout computed to be: 180
117: date       time         ( uptime  ) [ thread name/id ]                   file:line     v| 
117: 2021-04-13 11:17:49.759 (   0.003s) [main thread     ]             loguru.cpp:591   Info| arguments: UnitTestError -v INFO --data-dir=/Users/4nt/vtk-m/data/data --baseline-dir=/Users/4nt/vtk-m/data/baseline --write-dir=/Users/4nt/vtk-m/build
117: 2021-04-13 11:17:49.759 (   0.003s) [main thread     ]             loguru.cpp:594   Info| Current dir: /Users/4nt/vtk-m/build/vtkm/cont/testing
117: 2021-04-13 11:17:49.759 (   0.003s) [main thread     ]             loguru.cpp:596   Info| stderr verbosity: 0
117: 2021-04-13 11:17:49.759 (   0.003s) [main thread     ]             loguru.cpp:597   Info| -----------------------------------
117: 2021-04-13 11:17:49.761 (   0.005s) [main thread     ]      UnitTestError.cxx:67    Info| Check base error messages
117: Error at ../vtkm/cont/testing/UnitTestError.cxx:57:ValidateError
117:    Expected more entries in the stack frame
117: 4       0x7fff598723d5 start + 1
117: 3          0x107c5e5aa main + 362
117: 2          0x10805368e UnitTestError(int, char**) + 30
117: 1          0x107c5e9e6 int vtkm::cont::testing::Testing::Run<void (*)()>(void (*)(), int&, char**) + 118
117: 0          0x1080537ac (anonymous namespace)::DoErrorTest() + 268 (count > 5)
117: 2021-04-13 11:17:49.764 (   0.008s) [main thread     ]             loguru.cpp:471   Info| atexit
8/8 Test #117: UnitTestError ..........................***Failed    0.01 sec

It appears that the stack depth of > 5 was just an observation and not part of the correctness of this test.