Fix test_pkey_dh.rb in FIPS. #902

Job	Run time
openssl-1.0.2u	2m 23s
openssl-1.1.0l	1m 41s
openssl-1.1.1w	1m 43s
openssl-3.0.12	2m 52s
openssl-3.1.4	2m 52s
libressl-3.1.5	2m 7s
libressl-3.2.7	2m 20s
libressl-3.3.6	2m 15s
libressl-3.4.3	2m 14s
libressl-3.5.3	2m 15s
libressl-3.6.3	2m 14s
libressl-3.7.3	2m 18s
libressl-3.8.1	2m 16s
openssl-3.0.12 fips	2m 45s
openssl-3.1.4 fips	2m 49s
openssl-head	3m 36s
openssl-head fips	3m 30s
ruby-versions / Generate Ruby versions	3s
ubuntu-22.04 2.7	58s
ubuntu-22.04 3.0	54s
ubuntu-22.04 3.1	58s
ubuntu-22.04 3.2	57s
ubuntu-22.04 head	56s
ubuntu-22.04 truffleruby	1m 32s
ubuntu-22.04 truffleruby-head	1m 43s
ubuntu-20.04 2.7	56s
ubuntu-20.04 3.0	52s
ubuntu-20.04 3.1	51s
ubuntu-20.04 3.2	51s
ubuntu-20.04 head	51s
ubuntu-20.04 truffleruby-head	1m 30s
macos-latest 2.7	1m 50s
macos-latest 3.0	1m 9s
macos-latest 3.1	1m 27s
macos-latest 3.2	56s
macos-latest head	1m 8s
macos-latest truffleruby-head	2m 39s
windows-latest 2.7	5m 0s
windows-latest 3.0	4m 47s
windows-latest 3.1	4m 40s
windows-latest 3.2	4m 32s
windows-latest ucrt	4m 11s
windows-latest mswin	2m 22s
	1h 30m 43s

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