lib: tst_has_slow_kconfig() cache the result #349

Job	Run time
job (debian:stable, gcc, i386)	8m 21s
job (debian:stable, ppc64el, powerpc64le-linux-gnu-gcc, 1, out, cross-compile)	4m 53s
job (debian:stable, arm64, aarch64-linux-gnu-gcc, 1, out, cross-compile)	5m 0s
job (debian:stable, s390x, s390x-linux-gnu-gcc, 1, out, cross-compile)	4m 45s
job (alpine:latest, gcc, asciidoctor)	7m 46s
job (debian:stable, gcc, out, minimal)	5m 57s
job (opensuse/archive:42.2, gcc)	10m 27s
job (quay.io/centos/centos:stream9, gcc, asciidoc-pdf, out)	11m 35s
job (debian:testing, gcc, asciidoctor-pdf)	11m 18s
job (debian:oldstable, clang)	12m 35s
job (fedora:latest, clang, 1, asciidoctor-pdf)	8m 27s
job (opensuse/leap:latest, gcc, asciidoc-pdf)	8m 27s
job (debian:oldstable, gcc, asciidoctor)	10m 56s
job (debian:testing, clang, asciidoctor-pdf)	12m 26s
job (ubuntu:jammy, gcc, asciidoctor, out)	8m 0s
job (ubuntu:bionic, gcc, asciidoc-pdf)	12m 1s
	2h 22m 54s

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