Improve test coverage for xinspect.cpp

[anutosh491]

These set of lines stand untested right now (go through comment below)

#269 (comment)

Our goal is to have as much coverage for tests as possible. Would be great if this can be addressed by adding tests in https://github.com/compiler-research/xeus-cpp/blob/main/test%2Ftest_interpreter.cpp