releasetools: Fix an issue with pubkey extraction.

When calling 'openssl x509 -pubkey' to extract the public key from a
certificate, openssl 1.0 and 1.1 handle the '-out' parameter
differently. openssl 1.0 doesn't write the output into the specified
filename, which leads to the payload verification failure in
check_ota_package_signature.VerifyAbOtaPayload(). This CL addresses
the issue by always collecting the output from stdout instead.

It also refactors the two copies into common.ExtractPublicKey(), and
adds unittest. get_testdata_dir() is moved into test_utils.py that holds
common utils for running the unittests.

Bug: 72884343
Test: python -m unittest test_common
Test: python -m unittest test_ota_from_target_files
Test: Run sign_target_files_apks with '--replace_ota_keys' on marlin
      target_files zip. Check the payload pubkey replacement.
Test: Trigger the tests with forrest, and tests no longer fail on
      machines with openssl 1.0.1.
Change-Id: Ib0389b360f064053e9aa7cc0546d718e7b23003b