[mlir][bufferize][NFC] Fix FileCheck capture

One of the test cases matched IR from a subsequent test case. For this reason, the test case appeared to pass while it is actually broken.

This change does not fix the test case itself. It will be fixed when we overhaul the buffer deallocation implementation. (The memory leak in this test case is an edge case.)

Differential Revision: https://reviews.llvm.org/D135046