explicitly closed fastapi test client

[evvaaaa]

We'd get the following failure around 50% of test runs -

FAILED tests/backends/tango/test_dsr.py::TestTangoDevice::test_big_enum - pytest.PytestUnraisableExceptionWarning: Exception ignored in: <socket.socket fd=-1, family=1, type=1, proto=0>

Traceback (most recent call last):
  File "/dls_sw/apps/python/miniforge/4.10.0-0/envs/python3.11/lib/python3.11/unittest/mock.py", line 2124, in _mock_set_magics
    setattr(_type, entry, MagicProxy(entry, self))
                          ^^^^^^^^^^^^^^^^^^^^^^^
ResourceWarning: unclosed <socket.socket fd=28, family=1, type=1, proto=0>

This wasn't a problem in the big enum itself, commenting out test_big_enum will simply move the failure to the next test.

Explicitly closing the test client seems to do the trick.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.74%. Comparing base (9d3d9bf) to head (18e9909).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #92   +/-   ##
=======================================
  Coverage   86.74%   86.74%           
=======================================
  Files          25       25           
  Lines        1056     1056           
=======================================
  Hits          916      916           
  Misses        140      140           

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[GDYendell]

Looks good, assuming TestClient does the right thing.