Rename extentBuf, extentView and offsetView

[j-stephan]

This PR renames a few variables inside ViewSubViewTest.cpp and ViewPlainPtrTest.cpp:

• extentBuf becomes bufExtent
• extentView becomes viewExtent
• offsetView becomes viewOffset

This is done to fix the following MSVC warning which breaks in #2107:

D:\a\alpaka\alpaka\test\unit\mem\view\src\ViewPlainPtrTest.cpp(37,20): warning C4459: declaration of 'extentBuf' hides global declaration [D:\a\alpaka\alpaka\build\test\unit\mem\view\memViewTest.vcxproj]

I renamed extentView and offsetView to keep the naming inside the affected functions consistent.