mail-filter/rspamd: require <doctest-2.4.7 for now

At least until the rspamd/doctest maintainers have a look at this.

Also move to DEPEND as this is just a single .h file and doubt
this is used as runtime (revbump is to update this).

Bug: https://bugs.gentoo.org/830624
Signed-off-by: Ionen Wolkens <ionen@gentoo.org>

mail-filter/rspamd/rspamd-3.1-r1.ebuild

@@ -1,4 +1,4 @@
-# Copyright 1999-2021 Gentoo Authors
+# Copyright 1999-2022 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
 
 EAPI=7
@@ -34,7 +34,6 @@ RDEPEND="${LUA_DEPS}
 	acct-user/rspamd
 	app-arch/zstd:=
 	dev-db/sqlite:3
-	dev-cpp/doctest
 	dev-libs/glib:2
 	dev-libs/icu:=
 	dev-libs/libev
@@ -51,7 +50,9 @@ RDEPEND="${LUA_DEPS}
 	dev-libs/openssl:0=[-bindist(-)]
 	pcre2? ( dev-libs/libpcre2:=[jit=] )
 	!pcre2? ( dev-libs/libpcre[jit=] )"
-DEPEND="${RDEPEND}"
+DEPEND="
+	${RDEPEND}
+	<dev-cpp/doctest-2.4.7"
 BDEPEND="
 	dev-util/ragel
 	virtual/pkgconfig