unittests: skip some tests on AL2

[kavon]

It's blocking the Swift CI job https://ci.swift.org/job/oss-swift-package-amazon-linux-2-aarch64/

[kavon]

@swift-ci smoke test

[kavon]

preset=buildbot_amazon_linux_2
@swift-ci Please test with preset Amazon Linux 2

[kavon]

@swift-ci Please Build Toolchain Amazon Linux 2

[kavon]

@swift-ci smoke test

[kavon]

@swift-ci Please Build Toolchain Amazon Linux 2

[kavon]

ASL2 toolchain fails due to some unrelated LLDB issue:

16:16:22  In file included from /home/build-user/llvm-project/lldb/source/Plugins/ScriptInterpreter/Python/Interfaces/ScriptedThreadPythonInterface.cpp:17:
16:16:22  /home/build-user/llvm-project/lldb/source/Plugins/ScriptInterpreter/Python/Interfaces/../lldb-python.h:53:1: error: static assertion failed due to requirement '((3 << 24) | (7 << 16) | (16 << 8) | (15 << 4) | (0 << 0)) >= 50855936': LLDB requires at least Python 3.8
16:16:22  static_assert(PY_VERSION_HEX >= 0x03080000,
16:16:22  ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
16:16:22  1 error generated.