Fix test_pkey_dh.rb in FIPS. #903

Job	Run time
openssl-1.0.2u	2m 30s
openssl-1.1.0l	1m 33s
openssl-1.1.1w	1m 51s
openssl-3.0.12	2m 44s
openssl-3.1.4	2m 53s
libressl-3.1.5	2m 20s
libressl-3.2.7	2m 13s
libressl-3.3.6	2m 22s
libressl-3.4.3	2m 14s
libressl-3.5.3	2m 14s
libressl-3.6.3	2m 16s
libressl-3.7.3	2m 39s
libressl-3.8.1	2m 23s
openssl-3.0.12 fips	2m 42s
openssl-3.1.4 fips	2m 48s
openssl-head	3m 31s
openssl-head fips	3m 24s
ruby-versions / Generate Ruby versions	2s
ubuntu-22.04 2.7	59s
ubuntu-22.04 3.0	51s
ubuntu-22.04 3.1	1m 1s
ubuntu-22.04 3.2	57s
ubuntu-22.04 head	56s
ubuntu-22.04 truffleruby	1m 36s
ubuntu-22.04 truffleruby-head	1m 42s
ubuntu-20.04 2.7	57s
ubuntu-20.04 3.0	1m 3s
ubuntu-20.04 3.1	1m 10s
ubuntu-20.04 3.2	50s
ubuntu-20.04 head	51s
ubuntu-20.04 truffleruby-head	1m 32s
macos-latest 2.7	1m 27s
macos-latest 3.0	1m 41s
macos-latest 3.1	58s
macos-latest 3.2	58s
macos-latest head	58s
macos-latest truffleruby-head	4m 29s
windows-latest 2.7	4m 57s
windows-latest 3.0	4m 31s
windows-latest 3.1	4m 33s
windows-latest 3.2	5m 24s
windows-latest ucrt	4m 22s
windows-latest mswin	2m 39s
	1h 34m 1s

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