autest: skip tls_engine_abort test when plugin is absent

[maskit]

Description

tls_engine_abort.test.py calls Test.PrepareTestPlugin(async_handshake, ts, '-delay-ms=2000')
unconditionally. On non-OpenSSL builds (e.g. BoringSSL) async_handshake.so is
never built, so this raises ValueError: PrepareTestPlugin: file does not exist: ... and the test errors out instead of skipping cleanly.

Test.SkipUnless(...) only registers skip conditions for the framework to
check later — it does not halt execution of the rest of the test script. So
even though the file already guards on:

Test.SkipUnless(
    Condition.HasOpenSSLVersion('1.1.1'),
    Condition.IsOpenSSL(),
    Condition(lambda: os.path.isfile(async_handshake), async_handshake + " not found."),
)

the Test.PrepareTestPlugin(...) call still runs at load time regardless.

This is the same gap #13372 fixed in tls_async_handshake.test.py.
tls_engine_abort.test.py was added afterward from that file as a template
but didn't carry the guard over. This PR applies the same fix: guard the call
on file existence so the test skips cleanly on non-OpenSSL builds.

Test plan

• Ran the autest on a non-OpenSSL (BoringSSL) build; test now skips instead
  of erroring.
• Ran the autest on an OpenSSL build; test still runs and passes as before.

[Copilot]

Pull request overview

This PR fixes an AuTest execution-time failure in tls_engine_abort.test.py when the async_handshake.so test plugin is not built (e.g., non-OpenSSL builds such as BoringSSL). It ensures the test script does not raise at import/load time and instead allows the framework to skip the test cleanly.

Changes:

• Guard Test.PrepareTestPlugin(async_handshake, ...) behind os.path.isfile(async_handshake) so it is only executed when the plugin exists.

[cmcfarlen]

LGTM. Same gap and same fix as #13372 (caf9c87), which applied the identical if os.path.isfile(async_handshake): guard to tls_async_handshake.test.py; this file was derived from that one before the guard landed.

The reasoning about Test.SkipUnless is right: it only registers conditions the framework evaluates later, so the Condition(lambda: os.path.isfile(async_handshake), ...) already in the file does nothing to stop PrepareTestPlugin from running at load time and raising ValueError. Guarding the call keeps the existing skip condition as the thing that actually reports the skip.

[cmcfarlen]

Cherry-picked to the 10.2.x branch as 6fb80a1 for the 10.2.0 release.