Fix test_pkey_dh.rb in FIPS.

We use dh2048_ffdhe2048.pem file (DH 2048 bits) instead of dh1024.pem file in
both non-FIPS and FIPS cases. Because the following command fails to generate
the pem file with 1024 bits. And the OpenSSL FIPS 140-2 security policy
document explains the DH public keys are allowed from 2048 bits.[1]

```
$ OPENSSL_CONF=/home/jaruga/.local/openssl-3.3.0-dev-fips-debug-1aa08644ec/ssl/openssl_fips.cnf \
  /home/jaruga/.local/openssl-3.3.0-dev-fips-debug-1aa08644ec/bin/openssl \
  dhparam -out dh1024.pem 1024
Generating DH parameters, 1024 bit long safe prime
dhparam: Generating DH key parameters failed
```

The dh2048_ffdhe2048.pem file was created by the following command with the
OpenSSL FIPS configuration file. The logic to generate the DH pem file is
different between non-FIPS and FIPS cases. In FIPS, it seems that the command
always returns the text defined as ffdhe2048 in the FFDHE groups in RFC 7919
unlike non-FIPS.[2]

As the generated pem file is a normal and valid PKCS#3-style group parameter, we
use the file for the non-FIPS case too.

```
$ OPENSSL_CONF=/home/jaruga/.local/openssl-3.3.0-dev-fips-debug-1aa08644ec/ssl/openssl_fips.cnf \
  /home/jaruga/.local/openssl-3.3.0-dev-fips-debug-1aa08644ec/bin/openssl \
  dhparam -out dh2048_ffdhe2048.pem 2048
```

Note that the hard-coded PEM-encoded string in the `test_DHparams` is
intentional to avoid modifying the content unintentionally.

* [1] https://www.openssl.org/source/ - OpenSSL 3.0.8 FIPS 140-2 security
  policy document page 25, Table 10 – Public Keys - DH Public
  - DH (2048/3072/4096/6144/8192) public key agreement key
* [2] RFC7919 - Appendix A.1: ffdhe2048
  https://www.rfc-editor.org/rfc/rfc7919#appendix-A.1

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