Test: support multiple assignments for `@testset let` (#50151)
Nested `ContextTestset` is supported, so we can stack it when there are
multiple assignments in a given `let` block.
```julia
julia> @testset let logi = log(im), op = !iszero
@test imag(logi) == π/2
@test op(real(logi))
end
Test Failed at none:3
Expression: !(iszero(real(logi)))
Context: logi = 0.0 + 1.5707963267948966im
op = !iszero
ERROR: There was an error during testing
```
