Fix test_pkey_dh.rb in FIPS. #900

Job	Run time
openssl-1.0.2u	2m 24s
openssl-1.1.0l	1m 34s
openssl-1.1.1w	1m 42s
openssl-3.0.12	2m 48s
openssl-3.1.4	2m 53s
libressl-3.1.5	2m 5s
libressl-3.2.7	2m 10s
libressl-3.3.6	2m 19s
libressl-3.4.3	2m 16s
libressl-3.5.3	2m 26s
libressl-3.6.3	2m 14s
libressl-3.7.3	2m 15s
libressl-3.8.1	2m 11s
openssl-3.0.12 fips	2m 45s
openssl-3.1.4 fips	2m 49s
openssl-head	3m 34s
openssl-head fips	3m 24s
ruby-versions / Generate Ruby versions	4s
ubuntu-22.04 2.7	57s
ubuntu-22.04 3.0	55s
ubuntu-22.04 3.1	56s
ubuntu-22.04 3.2	59s
ubuntu-22.04 head	55s
ubuntu-22.04 truffleruby	1m 35s
ubuntu-22.04 truffleruby-head	1m 46s
ubuntu-20.04 2.7	54s
ubuntu-20.04 3.0	53s
ubuntu-20.04 3.1	52s
ubuntu-20.04 3.2	51s
ubuntu-20.04 head	48s
ubuntu-20.04 truffleruby-head	1m 31s
macos-latest 2.7	1m 49s
macos-latest 3.0	1m 33s
macos-latest 3.1	1m 17s
macos-latest 3.2	1m 54s
macos-latest head	1m 13s
macos-latest truffleruby-head	2m 21s
windows-latest 2.7	5m 25s
windows-latest 3.0	4m 32s
windows-latest 3.1	5m 1s
windows-latest 3.2	5m 58s
windows-latest ucrt	4m 9s
windows-latest mswin	2m 41s
	1h 33m 38s

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