apacheGH-15192: [C++] Bring back case_when tests for union types (a…

…pache#39308)

### Rationale for this change

Bring back the problematic test case of random `case_when` on union(bool, string) type. This case used to fail. However apache#36018 already addressed the issue.

More information about how it used to fail, please refer to apache#15192 (comment).

### What changes are included in this PR?

Bring back the test code.

### Are these changes tested?

Yes, the change is the test.

### Are there any user-facing changes?

No.

* Closes: apache#15192

Authored-by: zanmato <zanmato1984@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>

cpp/src/arrow/compute/kernels/scalar_if_else_test.cc

@@ -2485,16 +2485,14 @@ TEST(TestCaseWhen, UnionBoolString) {
   }
 }
 
-// FIXME(GH-15192): enabling this test produces test failures
-
-// TEST(TestCaseWhen, UnionBoolStringRandom) {
-//   for (const auto& type : std::vector<std::shared_ptr<DataType>>{
-//            sparse_union({field("a", boolean()), field("b", utf8())}, {2, 7}),
-//            dense_union({field("a", boolean()), field("b", utf8())}, {2, 7})}) {
-//     ARROW_SCOPED_TRACE(type->ToString());
-//     TestCaseWhenRandom(type);
-//   }
-// }
+TEST(TestCaseWhen, UnionBoolStringRandom) {
+  for (const auto& type : std::vector<std::shared_ptr<DataType>>{
+           sparse_union({field("a", boolean()), field("b", utf8())}, {2, 7}),
+           dense_union({field("a", boolean()), field("b", utf8())}, {2, 7})}) {
+    ARROW_SCOPED_TRACE(type->ToString());
+    TestCaseWhenRandom(type);
+  }
+}
 
 TEST(TestCaseWhen, DispatchBest) {
   CheckDispatchBest("case_when", {struct_({field("", boolean())}), int64(), int32()},