GH-45455: [GLib] Fix returns positive memory-pool utilization (#45456)

### Rationale for this change

Due to byte_allocated size returning 0, The TestMemoryPool-only test failed.
This issue did not occur when running other tests together because the other tests use the memory pool.

### What changes are included in this PR?

Allocate a ResizableBuffer in the TestMemoryPool test.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.

* GitHub Issue: #45455

Lead-authored-by: Hiroyuki Sato <hiroysato@gmail.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

Author: hiroyuki-sato

c_glib/test/test-memory-pool.rb

@@ -20,6 +20,8 @@ class TestMemoryPool < Test::Unit::TestCase
 
   def setup
     @memory_pool = Arrow::MemoryPool.default
+    # Our tests assume that some memory is allocated.
+    @buffer = Arrow::ResizableBuffer.new(1)
   end
 
   def test_bytes_allocated