BUG: Fix `ColorTableTestSpecialConditionChecker` component/pixel mix-up
BUG: Fix `ColorTableTestSpecialConditionChecker` component/pixel mix-up

It appears that `ColorTableTestSpecialConditionChecker<TComponent>()`
mixed up color component and pixel types, when it assigned the result of
two `GetColorComponent` calls to a `pixel` variable. `GetColorComponent`
returns a component value, rather than a pixel value.

This bug was found by declaring the `RGBPixel(const ComponentType &)`
constructor `explicit` (locally, temporarily), to detect accidental
implicit conversions from a component value to an `RGBPixel` value.

Follow-up to:
pull request https://github.com/InsightSoftwareConsortium/ITK/pull/3221
commit 9cfaf8fbdaba802e3b6406db300581e25ddec9ff
"STYLE: Rename `ColorTable` template parameter `TPixel` to `TComponent`"

InsightSoftwareConsortium/ITK,hjmjohnson/ITK,thewtex/ITK,BRAINSia/ITK,InsightSoftwareConsortium/ITK,richardbeare/ITK,Kitware/ITK,Kitware/ITK,BRAINSia/ITK,Kitware/ITK,BRAINSia/ITK,BRAINSia/ITK,Kitware/ITK,InsightSoftwareConsortium/ITK,thewtex/ITK,Kitware/ITK,hjmjohnson/ITK,thewtex/ITK,hjmjohnson/ITK,hjmjohnson/ITK,Kitware/ITK,richardbeare/ITK,BRAINSia/ITK,richardbeare/ITK,BRAINSia/ITK,Kitware/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,thewtex/ITK,thewtex/ITK,thewtex/ITK,hjmjohnson/ITK,richardbeare/ITK,richardbeare/ITK,richardbeare/ITK,BRAINSia/ITK,hjmjohnson/ITK,InsightSoftwareConsortium/ITK,InsightSoftwareConsortium/ITK,richardbeare/ITK,InsightSoftwareConsortium/ITK,thewtex/ITK