chore(langchain): disable flaky tests (#11511)

There appear to be stability issues with using snapshots and/or
LangChain in general.

There are failures in the mocked tests that look like:

```
builtins.AssertionError: assert 0 == 1
 +  where 0 = <MagicMock name='LLMObsSpanWriter().enqueue' id='127482146130048'>.call_count
 +    where <MagicMock name='LLMObsSpanWriter().enqueue' id='127482146130048'> = <MagicMock name='LLMObsSpanWriter()' id='127482147073440'>.enqueue
```

as well as failures with snapshot based tests:

```
builtins.Failed: At request <Request GET /test/session/snapshot >:
   At snapshot (token='tests.contrib.langchain.test_langchain_community.test_lcel_chain_simple'):
    - Directory: /go/src/github.com/DataDog/apm-reliability/dd-trace-py/tests/snapshots
    - CI mode: 1
    - Trace File: /go/src/github.com/DataDog/apm-reliability/dd-trace-py/tests/snapshots/tests.contrib.langchain.test_langchain_community.test_lcel_chain_simple.json
    - Stats File: /go/src/github.com/DataDog/apm-reliability/dd-trace-py/tests/snapshots/tests.contrib.langchain.test_langchain_community.test_lcel_chain_simple_tracestats.json
    At compare of 1 expected trace(s) to 0 received trace(s):
Did not receive expected traces: 'langchain.request'
```

While we investigate a more stable method of testing it makes sense to
disable the tests to avoid noise to our neighbours in the library :).

DOWN WITH FLAKY TESTS

[](https://datadoghq.atlassian.net/browse/MLOB-1920)

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