selftest: tpm2: Add Client.__del__() to close /dev/tpm* handle

The following output can bee seen when the test is executed:

  test_flush_context (tpm2_tests.SpaceTest) ... \
    /usr/lib64/python3.6/unittest/case.py:605: ResourceWarning: \
    unclosed file <_io.FileIO name='/dev/tpmrm0' mode='rb+' closefd=True>

An instance of Client does not implicitly close /dev/tpm* handle, once it
gets destroyed. Close the file handle in the class destructor
Client.__del__().

Fixes: 6ea3dfe1 ("selftests: add TPM 2.0 tests")
Cc: Shuah Khan <shuah@kernel.org>
Cc: linux-kselftest@vger.kernel.org
Cc: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>