tests/linux_kernel: Skip TestStackDepot unless test kmod is present

[sourabhjains]

TestStackDeport needs the drgn test kernel module, so if it is not present, skip the test.

Error reported:

ERROR: setUpClass (tests.linux_kernel.helpers.test_stackdepot.TestStackDepot)
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Traceback (most recent call last):
  File "/root/drgn/tests/linux_kernel/helpers/test_stackdepot.py", line 15, in setUpClass
    if not cls.prog["drgn_test_have_stackdepot"]:
KeyError: 'drgn_test_have_stackdepot'

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Ran 980 tests in 4.056s

[osandov]

Good catch, thank you.

[sourabhjains]

Thanks for accepting the patch.