Update incorrect_msg in has_expr when set_context/set_env used

[filipsch]

Currently, the context variable values or environment variable values are not included in the automatically generated message, which can be confusing.

[hermansje]

We should probably always mention the context/env when it is set (also when calling a function with check_call, rerunning a body with check_body().has_equal_output(), checking the value of an argument with check_args('x')...).