Test: support multiple assignments for @testset let (JuliaLang#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
```

stdlib/Test/src/Test.jl

@@ -1351,11 +1351,11 @@ function _check_testset(testsettype, testsetname)
 end
 
 """
- @testset [CustomTestSet] [option=val ...] ["description"] begin ... end
- @testset [CustomTestSet] [option=val ...] ["description \$v"] for v in (...) ... end
- @testset [CustomTestSet] [option=val ...] ["description \$v, \$w"] for v in (...), w in (...) ... end
- @testset [CustomTestSet] [option=val ...] ["description"] foo()
- @testset let v = (...) ... end
+ @testset [CustomTestSet] [options...] ["description"] begin test_ex end
+ @testset [CustomTestSet] [options...] ["description \$v"] for v in itr test_ex end
+ @testset [CustomTestSet] [options...] ["description \$v, \$w"] for v in itrv, w in itrw test_ex end
+ @testset [CustomTestSet] [options...] ["description"] test_func()
+ @testset let v = v, w = w; test_ex; end
 
 # With begin/end or function call
 
@@ -1380,7 +1380,7 @@ accepts three boolean options:
  This can also be set globally via the env var `JULIA_TEST_FAILFAST`.
 
 !!! compat "Julia 1.8"
- `@testset foo()` requires at least Julia 1.8.
+ `@testset test_func()` requires at least Julia 1.8.
 
 !!! compat "Julia 1.9"
  `failfast` requires at least Julia 1.9.
@@ -1436,6 +1436,9 @@ parent test set (with the context object appended to any failing tests.)
 !!! compat "Julia 1.9"
  `@testset let` requires at least Julia 1.9.
 
+!!! compat "Julia 1.10"
+ Multiple `let` assignements are supported since Julia 1.10.
+
 ## Examples
 ```jldoctest
 julia> @testset let logi = log(im)
@@ -1446,6 +1449,17 @@ Test Failed at none:3
  Expression: !(iszero(real(logi)))
  Context: logi = 0.0 + 1.5707963267948966im
 
+ERROR: There was an error during testing
+
+julia> @testset let logi = log(im), op = !iszero
+ @test imag(logi) == π/2
+ @test op(real(logi))
+ end
+Test Failed at none:3
+ Expression: op(real(logi))
+ Context: logi = 0.0 + 1.5707963267948966im
+ op = !iszero
+
 ERROR: There was an error during testing
 ```
 """
@@ -1477,30 +1491,46 @@ trigger_test_failure_break(@nospecialize(err)) =
 """
 Generate the code for an `@testset` with a `let` argument.
 """
-function testset_context(args, tests, source)
+function testset_context(args, ex, source)
  desc, testsettype, options = parse_testset_args(args[1:end-1])
  if desc !== nothing || testsettype !== nothing
  # Reserve this syntax if we ever want to allow this, but for now,
  # just do the transparent context test set.
  error("@testset with a `let` argument cannot be customized")
  end
 
- assgn = tests.args[1]
- if !isa(assgn, Expr) || assgn.head !== :(=)
- error("`@testset let` must have exactly one assignment")
+ let_ex = ex.args[1]
+
+ if Meta.isexpr(let_ex, :(=))
+ contexts = Any[let_ex.args[1]]
+ elseif Meta.isexpr(let_ex, :block)
+ contexts = Any[]
+ for assign_ex in let_ex.args
+ if Meta.isexpr(assign_ex, :(=))
+ push!(contexts, assign_ex.args[1])
+ else
+ error("Malformed `let` expression is given")
+ end
+ end
+ else
+ error("Malformed `let` expression is given")
  end
- assignee = assgn.args[1]
+ reverse!(contexts)
+
+ test_ex = ex.args[2]
 
- tests.args[2] = quote
- $push_testset($(ContextTestSet)($(QuoteNode(assignee)), $assignee; $options...))
+ ex.args[2] = quote
+ $(map(contexts) do context
+ :($push_testset($(ContextTestSet)($(QuoteNode(context)), $context; $options...)))
+ end...)
  try
- $(tests.args[2])
+ $(test_ex)
  finally
- $pop_testset()
+ $(map(_->:($pop_testset()), contexts)...)
  end
  end
 
- return esc(tests)
+ return esc(ex)
 end
 
 """