@testset for: avoid calling finish twice when it errors (#41715)

Here is a MWE:
```julia
julia> using Test
       @testset "a" for i=1:2
           @test i != 1
       end
a: Test Failed at REPL[3]:3
  Expression: i != 1
   Evaluated: 1 != 1
Stacktrace:
[...]
Test Summary: | Fail  Total
a             |    1      1
Test Summary: | Fail  Total
a             |    1      1
ERROR: Some tests did not pass: 0 passed, 1 failed, 0 errored, 0 broken.

caused by: Some tests did not pass: 0 passed, 1 failed, 0 errored, 0 broken.
```

The `finish` function is called twice, and for a toplevel testset, this means
throwing an error. This manifests in the above example by printing twice
the result of the testset (with "Test Summary"), and by having
"Error: ... caused by: ..." with the same message.

Co-authored-by: Kristoffer Carlsson <[email protected]>

stdlib/Test/src/Test.jl

@@ -1425,7 +1425,10 @@ function testset_forloop(args, testloop, source)
  # they can be handled properly by `finally` lowering.
  if !first_iteration
  pop_testset()
+ finish_errored = true
  push!(arr, finish(ts))
+ finish_errored = false
+
  # it's 1000 times faster to copy from tmprng rather than calling Random.seed!
  copy!(RNG, tmprng)
 
@@ -1447,6 +1450,7 @@ function testset_forloop(args, testloop, source)
  local arr = Vector{Any}()
  local first_iteration = true
  local ts
+ local finish_errored = false
  local RNG = default_rng()
  local oldrng = copy(RNG)
  local oldseed = Random.GLOBAL_SEED
@@ -1458,7 +1462,7 @@ function testset_forloop(args, testloop, source)
  end
  finally
  # Handle `return` in test body
- if !first_iteration
+ if !first_iteration && !finish_errored
  pop_testset()
  push!(arr, finish(ts))
  end