Restore support for checking for UndefVarError variable name in at-test_throws (#56231)

Fix #54082

Arguably this was a breaking change (as a consequence of
#51979). But regardless, it seems
like useful functionality to have a public API for testing that an
`UndefVarError` was thrown for the expected variable name (regardless of
scope). This is particularly useful if you don't know what the scope is
(for example, in my use-case i want to test that a specific
`UndefVarError` is thrown from a module with a `gensym`'d name).

Pre-v1.11 the syntax for this was
```julia
@test_throws UndefVarError(:x) foo()
```
but that stopped working in v1.11 when `UndefVarError` got a second
field (in fact in v1.11.1 this is an error when before it would pass)

This PR restores that functionality.

We might want to backport it to v1.11.x so that v1.11 isn't the only
version that doesn't support this.

(cherry picked from commit b0c1525)

Author: nickrobinson251

stdlib/Test/src/Test.jl

@@ -809,7 +809,11 @@ function do_test_throws(result::ExecutionResult, orig_expr, extype)
             if extype isa LoadError && !(exc isa LoadError) && typeof(extype.error) == typeof(exc)
                 extype = extype.error # deprecated
             end
-            if isa(exc, typeof(extype))
+            # Support `UndefVarError(:x)` meaning `UndefVarError(:x, scope)` for any `scope`.
+            # Retains the behaviour from pre-v1.11 when `UndefVarError` didn't have `scope`.
+            if isa(extype, UndefVarError) && !isdefined(extype, :scope)
+                success = exc isa UndefVarError && exc.var == extype.var
+            else isa(exc, typeof(extype))
                 success = true
                 for fld in 1:nfields(extype)
                     if !isequal(getfield(extype, fld), getfield(exc, fld))

stdlib/Test/test/runtests.jl

@@ -1710,3 +1710,20 @@ end
         @test occursin(expected, result)
     end
 end
+
+# Issue #54082
+module M54082 end
+@testset "@test_throws UndefVarError(:var)" begin
+    # Single-arg `UndefVarError` should match all `UndefVarError` for the
+    # same variable name, regardless of scope, to keep pre-v1.11 behaviour.
+    f54082() = var
+    @test_throws UndefVarError(:var) f54082()
+    # But if scope is set, then it has to match.
+    @test_throws UndefVarError(:var, M54082) M54082.var
+    let result = @testset NoThrowTestSet begin
+            # Wrong module scope
+            @test_throws UndefVarError(:var, Main) M54082.var
+        end
+        @test only(result) isa Test.Fail
+    end
+end