Read test cacert and keys from files

Change the way how the tests read CaCerts and private key. Now,
they are read from the files in test_keys directory. This is
in preparation for DoH tests because quiche can read them only
from file path.

Bug: 181642979
Test: cd packages/modules/DnsResolver && atest
Test: mts-tradefed run mts-dnsresolver-native
Change-Id: Ifc9813f22afe4d8780bdceb6d7a1a5d24717f5d5