Use libsoup3 for tests

Even when using the curl fetcher backend, libsoup is used for tests, but
either version can be used.  libsoup (2.y) is unmaintained, and unwanted
in RHEL 10.

ostree.spec

@@ -24,7 +24,7 @@ BuildRequires: pkgconfig(zlib)
 BuildRequires: pkgconfig(libcurl)
 BuildRequires: openssl-devel
 # The tests still require soup
-BuildRequires: pkgconfig(libsoup-2.4)
+BuildRequires: pkgconfig(libsoup-3.0)
 BuildRequires: libattr-devel
 # The tests require attr
 BuildRequires: attr
@@ -104,6 +104,7 @@ env NOCONFIGURE=1 ./autogen.sh
            --enable-gtk-doc \
            --with-selinux \
            --with-curl \
+           --with-soup3 \
            --with-openssl \
            %{?with_tests:--enable-installed-tests=exclusive} \
            --with-dracut=yesbutnoconf