reset GLOBAL_RNG state in/out at-testset for reproducibility (#24445)

This is a follow-up on the recently introduced `guardsrand` functionality,
first suggested at
JuliaLang/julia#16940 (comment).
Each `testset` block is now implicitly wrapped in a `guardsrand` block, which
is more user-friendly and more systematic (only few users know about
`guardsrand`).

These are essentially two new features:

1) "in": in base, tests are run with the global RNG randomly seeded,
   but the seed is printed in case of failure to allow reproducing the
   failure; but even if the failure occurs after many tests, when they
   alter the global RNG state, one has to re-run the whole file, which
   can be time-consuming; with this change, at the beginning of each
   `testset`, the global RNG is re-seeded with its own seed: this
   allows to re-run only the failing `testset`, which can be done
   easily in the REPL (the seeding occurs for each loop in a `testset for`;
   this also allows to re-arrange `testset`s in arbitrary order
   w.r.t. the global RNG;

2) "out": a `testset` leaves no tracks of its use of `srand` or `rand`
   (this "feature" should be less and less needed/useful with the
   generalization of the use of `testset` blocks).

Example:
```
@testset begin
   srand(123)
   rand()
   @testset for T in (Int, Float64)
       # here is an implicit srand(123), by 1)
       rand()
   end
   rand() # this value will not be affected if the sub-`testset` block
          # above is removed, or if another rand() call is added therein, by 2)
end
```

Note that guardsrand can't be used directly, as then the testset's body is
wrapped in a function, which causes problem with overwriting loop variable
("outer"), using `using`, defining new methods, etc. So we need to duplicate
guardsrand's logic.

Author: rfourquet

SuiteSparse/test/cholmod.jl

@@ -723,6 +723,7 @@ end
 end
 
 @testset "Check that Symmetric{SparseMatrixCSC} can be constructed from CHOLMOD.Sparse" begin
+    Int === Int32 && srand(124)
     A = sprandn(10, 10, 0.1)
     B = CHOLMOD.Sparse(A)
     C = B'B