[benchmark] Move fake autoreleasepool to TestUtils

[palimondo]

This workaround for writing cross platform benchmarks originally introduced by @gottesmm in #21905 is required in multiple performance tests.

Undeclared autoreleasepool was the reason for failing linux smoke test on #22648 during swiftpm compilation of benchmarks. This should fix it.

[palimondo]

@swift-ci please benchmark

[palimondo]

@swift-ci please smoke test

[palimondo]

@eeckstein @gottesmm Please review 🙏

[swift-ci]

Performance: -O

TEST	OLD	NEW	DELTA	RATIO
Regression
Breadcrumbs.CopyUTF16CodeUnits.Mixed	58	64	+10.3%	0.91x (?)

Performance: -Onone

TEST	OLD	NEW	DELTA	RATIO
Improvement
ObjectiveCBridgeStubFromNSString	986	915	-7.2%	1.08x (?)

How to read the data

The tables contain differences in performance which are larger than 8% and differences in code size which are larger than 1%.

If you see any unexpected regressions, you should consider fixing the
regressions before you merge the PR.

Noise: Sometimes the performance results (not code size!) contain false
alarms. Unexpected regressions which are marked with '(?)' are probably noise.
If you see regressions which you cannot explain you can try to run the
benchmarks again. If regressions still show up, please consult with the
performance team (@eeckstein).

Hardware Overview

  Model Name: Mac Pro
  Model Identifier: MacPro6,1
  Processor Name: 12-Core Intel Xeon E5
  Processor Speed: 2.7 GHz
  Number of Processors: 1
  Total Number of Cores: 12
  L2 Cache (per Core): 256 KB
  L3 Cache: 30 MB
  Memory: 64 GB

[ianpartridge]

Looks good to me - thanks @palimondo.

[gottesmm]

LGTM

[ianpartridge]

Can we merge this to unblock #22648?