Print more context for complex test conditions (#46138)
On some failures, e.g. in the cmdlineargs failure in [1],
printing the failing expression doesn't give us very much
information, because what we're doing is just testing a
small part of a larger structure, but to really diagnose
the failure, we'd really want to see the whole structure
(in this case the stderr output, not just the failure bool).
This introduces an additional `@testset` feature that lets
failures optionally include a context variable that gets printed
on failure. Example:
```
julia> @testset let v=(1,2,3)
@test v[1] == 1
@test v[2] == 3
end
Test Failed at REPL[8]:3
Expression: v[2] == 3
Evaluated: 2 == 3
Context: v = (1, 2, 3)
ERROR: There was an error during testing
```
The syntax here is `@testset let`, which was previously unused.
The testset is transparent and failures/successes are passed
through directly to the parent testset. In particular, using
this kind of testset does not cause additional nesting in
the default testset print.
[1] https://buildkite.com/julialang/julia-master/builds/14160#01822311-84f0-467a-a8af-a5d751f2c6ab/448-728
