Use runuser instead of sudo for test-suite user changes

We don't need or want any of the sudo fanciness and privilege elevation
here: we're already root inside the container and just want to run
things as a different user. runuser is more than adequate for this
purpose.

One such peculiarity is that apparently sudo re-expands its command,
so \$PWD got evaluated to the PWD of the new shell. runuser does no such
thing, but we can just as well hardcode a directory, $PWD is nothing
special in this case.

Consolidate the lone runuser rpmdb export test to use runroot_user too
now that it can.

tests/atlocal.in

@@ -139,7 +139,7 @@ runroot_user()
             shift 2
         ;;
     esac
-    runroot --new-session sudo -iu $RPMUSER "$@"
+    runroot --new-session /sbin/runuser -u $RPMUSER -- "$@"
 }
 
 snapshot prune rpmtests.dir/*/tree

tests/rpmdb.at

@@ -99,9 +99,7 @@ runroot rpm -U /data/RPMS/hlinktest-1.0-1.noarch.rpm
 [])
 
 RPMTEST_CHECK([
-# Need to pass the dbpath explicitly since we're not going through run/runroot
-dbpath=$(rpm --eval "%_dbpath")
-runroot /sbin/runuser -u nobody -- rpmdb --dbpath ${dbpath} --exportdb > hdr.list
+runroot_user -n nobody rpmdb --exportdb > hdr.list
 ],
 [0],
 [],

tests/rpmi.at

@@ -1175,18 +1175,20 @@ RPMTEST_USER
 
 runroot rpmbuild --quiet -bb /data/SPECS/rootfs.spec
 
+USERDIR=/tmp/mydir
+
 RPMTEST_CHECK([
 runroot_user \
-  rpm -U --prefix \$PWD/root --dbpath \$PWD/rpmdb \
+  rpm -U --prefix ${USERDIR}/root --dbpath ${USERDIR}/rpmdb \
   /build/RPMS/noarch/rootfs-1.0-1.noarch.rpm
 ],
 [0],
 [],
 [])
 
 RPMTEST_CHECK([
-runroot_user rpm -e --dbpath \$PWD/rpmdb rootfs
-runroot_user test ! -d \$PWD/root
+runroot_user rpm -e --dbpath ${USERDIR}/rpmdb rootfs
+runroot_user test ! -d ${USERDIR}/root
 ],
 [0],
 [],