Introduced fixed certificates for tests

[dw-kihara]

I discovered the tests were failing because the tests check the contents of certificates, which changes frequently.
To address this, I introduced cacerts_test_data/0 which returns the fixed certificates for testing.

Although cacerts/0 could return the fixed certificats during tests,
I kept it to return the content of priv/cacerts.pem to ensure it provides the correct certificates.

I also modified certifi_pt.erl to transform both functions and to make Dialyzer happy.

[dw-kihara]

Initially, I thought this number could be generated by parsing priv/cacerts.pem.
However, I realized this approach would weaken the test because parsing the file to verify it is properly parsed sounds somewhat circular.