testsuite: add modinfo pkcs7 signature test

testsuite: add modinfo pkcs7 signature test

Use the same approach to generate the signed module, like in the
old signature test: just append the pregenerated binary signature
to the module (the signature check will fail).

In case of need of generating correct signature, from the linux
kernel makefiles (certs/Makefile) it could be like:

$ openssl req -new -nodes -utf8  -sha256 -days 36500 -batch -x509
-config  ./x509.genkey -outform PEM -out signing_key.pem -keyout signing_key.pem
$ /lib/modules/$(uname -r)/build/scripts/extract-cert signing_key.pem signing_key.x509
$ /lib/modules/$(uname -r)/build/scripts/sign-file sha256 signing_key.pem signing_key.x509 module.ko

where x509.genkey is:

```
[ req ]
default_bits = 4096
distinguished_name = req_distinguished_name
prompt = no
string_mask = utf8only
x509_extensions = myexts

[ req_distinguished_name ]
CN = Build time autogenerated kernel key

[ myexts ]
basicConstraints=critical,CA:FALSE
keyUsage=digitalSignature
subjectKeyIdentifier=hash
authorityKeyIdentifier=keyid
```

Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>