COMP: Use signed char for SobelOperator convolution test

In 03c8176 a concept check was added
that the sobel operator uses a signed pixel type. On Linux aarch64,

❯ c++ --version
c++ (Debian 12.2.0-14) 12.2.0

❯ uname -a
Linux deb 6.1.0-18-arm64 #1 SMP Debian 6.1.76-1 (2024-02-01) aarch64 GNU/Linux

char is unsigned, which causes the concept check to fail. Switch `char`
to `signed char`.

Author: thewtex

Modules/Core/Common/test/itkSobelOperatorImageConvolutionTest.cxx

@@ -130,7 +130,8 @@ itkSobelOperatorImageConvolutionTest(int argc, char * argv[])
     /*
      * Demonstrate that signed types do work with SobelOperator
      */
-    return_status += DoSimpleConvolutionTest<char, 2>(direction, "char");
+    // Note: `char` can be unsigned on some platforms.
+    return_status += DoSimpleConvolutionTest<signed char, 2>(direction, "signed char");
     return_status += DoSimpleConvolutionTest<short, 2>(direction, "short");
     return_status += DoSimpleConvolutionTest<int, 2>(direction, "int");
     return_status += DoSimpleConvolutionTest<long, 2>(direction, "long");