STYLE: Replace char with signed char as itk::Image argument in tests

[N-Dekker]

Replaced itk::Image<char with itk::Image<signed char, in all "itk*Test*.cxx" source files. Removed pixel type alias InputDataType = char from itkAntiAliasBinaryImageFilterTest, and directly used signed char instead.

The use of char as TPixel argument in tests appears to triggers test failures, when compiling with an unsigned default char type (GCC option -funsigned-char or MSVC option /J), for example:

[ RUN      ] FrequencyIterators.Even2D
Unequal images, with 29 unequal pixels
test/itkFrequencyIteratorsGTest.cxx:260: Failure
Value of: fullAndHermitian
Actual: false
Expected: true

It appears clearer to explicitly specify that the pixel type should be signed, by using signed char instead.

---

• Those test failures appeared with an experimental pull request: WIP: [Do not merge] Change the default char type to unsigned #5455
• This pull request is inspired by a comment from Bradley (@blowekamp) at Let IOComponent::CHAR represents signed char #5450 (comment)
• Various coding guidelines recommend using signed char, rather than plain char, for numeric values, for example https://wiki.sei.cmu.edu/confluence/display/c/INT07-C.+Use+only+explicitly+signed+or+unsigned+char+type+for+numeric+values and https://rules.sonarsource.com/cpp/tag/confusing/RSPEC-810/

[blowekamp]

Looks good. With the IO changes, the IO related tests should clearly be signed char. In the level sets the char images are used for mask/label, in this case I am not sure the sign-ness was intended, but I don' think it matters.