Run the "diff of expected output" in a separate AT_CHECK instead of
using the 'run-if-pass' section; this correctly detects failure.

(ab)use --pass-fd to avoid the passphrase prompt

Use AT_TESTED
Modify the PATH to the build dir instead of hardcoding NETPGP* vars.

wrap "copy the test file" in AT_CHECK

More tests for netpgp

Add tests for detached signature creation and verification, and for
encryption and decryption via netpgp.