Add context to `errorshow` `except_str` tests (#56199)
With this, the error message in
https://buildkite.com/julialang/julia-master/builds/41054#019294ca-e2c5-41f2-a897-e2959715f154
would become
```julia
Error in testset errorshow:
Test Failed at /home/jishnu/juliaPR/usr/share/julia/test/errorshow.jl:226
Expression: typeof(err) === $(Expr(:escape, :MethodError))
Evaluated: StackOverflowError === MethodError
Context: expr = :(+())
```
Having the failing expression displayed makes it easier to locate the
source of the error.
