apacheGH-39050: [C++] Use Cast() instead of CastTo() for Timestamp Sc…

…alar in test (apache#39060)

### Rationale for this change

Remove legacy code

### What changes are included in this PR?

* Replace the legacy scalar CastTo implementation for Timestamp Scalar in test. The cast function already supports casting between different timestamp types. We just need to allow for the possibility of data loss by using `CastOptions::Unsafe()`. 

### Are these changes tested?

Yes. It is passed by existing test cases.

### Are there any user-facing changes?

No.

* Closes: apache#39050

Authored-by: Hyunseok Seo <[email protected]>
Signed-off-by: Sutou Kouhei <[email protected]>

cpp/src/arrow/scalar_test.cc

@@ -30,6 +30,7 @@
 #include "arrow/array.h"
 #include "arrow/array/util.h"
 #include "arrow/buffer.h"
+#include "arrow/compute/cast.h"
 #include "arrow/memory_pool.h"
 #include "arrow/scalar.h"
 #include "arrow/status.h"
@@ -40,6 +41,9 @@
 
 namespace arrow {
 
+using compute::Cast;
+using compute::CastOptions;
+
 using internal::checked_cast;
 using internal::checked_pointer_cast;
 
@@ -862,9 +866,9 @@ TEST(TestTimestampScalars, MakeScalar) {
 
 TEST(TestTimestampScalars, Cast) {
   auto convert = [](TimeUnit::type in, TimeUnit::type out, int64_t value) -> int64_t {
-    auto scalar =
-        TimestampScalar(value, timestamp(in)).CastTo(timestamp(out)).ValueOrDie();
-    return internal::checked_pointer_cast<TimestampScalar>(scalar)->value;
+    EXPECT_OK_AND_ASSIGN(auto casted, Cast(TimestampScalar(value, timestamp(in)),
+                                           timestamp(out), CastOptions::Unsafe()));
+    return internal::checked_pointer_cast<TimestampScalar>(casted.scalar())->value;
   };
 
   EXPECT_EQ(convert(TimeUnit::SECOND, TimeUnit::MILLI, 1), 1000);