Fix test_pkey_dh.rb in FIPS. #890

Job	Run time
openssl-1.0.2u	2m 25s
openssl-1.1.0l	1m 31s
openssl-1.1.1w	1m 41s
openssl-3.0.12	4m 20s
openssl-3.1.4	4m 27s
libressl-3.1.5	3m 53s
libressl-3.2.7	3m 5s
libressl-3.3.6	2m 15s
libressl-3.4.3	3m 7s
libressl-3.5.3	2m 15s
libressl-3.6.3	2m 14s
libressl-3.7.3	3m 7s
libressl-3.8.1	2m 11s
openssl-3.0.12 fips	2m 45s
openssl-3.1.4 fips	2m 50s
openssl-head	5m 32s
openssl-head fips	8m 7s
ruby-versions / Generate Ruby versions	3s
ubuntu-22.04 2.7	1m 27s
ubuntu-22.04 3.0	55s
ubuntu-22.04 3.1	1m 12s
ubuntu-22.04 3.2	57s
ubuntu-22.04 head	1m 7s
ubuntu-22.04 truffleruby	2m 4s
ubuntu-22.04 truffleruby-head	1m 42s
ubuntu-20.04 2.7	57s
ubuntu-20.04 3.0	53s
ubuntu-20.04 3.1	53s
ubuntu-20.04 3.2	49s
ubuntu-20.04 head	56s
ubuntu-20.04 truffleruby-head	1m 32s
macos-latest 2.7	1m 40s
macos-latest 3.0	1m 2s
macos-latest 3.1	1m 4s
macos-latest 3.2	1m 33s
macos-latest head	1m 39s
macos-latest truffleruby-head	3m 9s
windows-latest 2.7	4m 19s
windows-latest 3.0	3m 57s
windows-latest 3.1	4m 28s
windows-latest 3.2	4m 52s
windows-latest ucrt	6m 30s
windows-latest mswin	3m 11s
	1h 48m 36s

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