### What changes were proposed in this pull request?
This PR updates `TestMemoryManager`, a class used only in unit tests, to override the `getExecutionMemoryUsageForTask()` and `releaseAllExecutionMemoryForTask()` methods. I also `synchronized` its state-accessing methods (to make the class thread-safe) and added some additional assertions to guard against freeing memory memory than has been allocated.
### Why are the changes needed?
Spark uses a `TestMemoryManager` class to mock out memory manager functionality in tests, allowing test authors to exercise control over certain behaviors (e.g. to simulate OOMs).
Several of Spark's test suites have memory-leak detection to ensure that all allocated memory is cleaned up at the end of each test case; this helps to guard against bugs that could cause production memory leaks. For example, see `testWithMemoryLeakDetection` in `UnsafeFixedWidthAggregationMapSuite`.
Unfortunately, however, this leak-detection logic is broken for tests which use TestMemoryManager because it does not override the `getExecutionMemoryUsageForTask()` method that is used by the leak-detection checks.
This PR fixes that problem, thereby strengthening our existing tests.
I spotted this problem while reviewing #25953: I tried introducing a change to remove a `freePage()` call (purposely inducing a memory leak) but no tests failed.
### Does this PR introduce any user-facing change?
No.
### How was this patch tested?
Added a new `TestMemoryManagerSuite`, with tests covering `TestMemoryManager` itself.
Ran a subset of existing tests on my laptop and uncovered a bug in one test's `free()` calls, plus missing cleanup calls in another test suite; both of these issues are fixed in this PR.
Closes#25985 from JoshRosen/SPARK-29310-testmemorymanager-getExecutionMemoryUsageForTask.
Lead-authored-by: Josh Rosen <rosenville@gmail.com>
Co-authored-by: joshrosen-stripe <48632449+joshrosen-stripe@users.noreply.github.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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