tests: fix with changed ABUILD_USERDIR

The test suite expects ABUILD_USERDIR to be "$HOME"/.abuild but that
could be overwritten in ABUILD_CONF. So use the fixed value instead of
relying on the default.

tests/abuild_keygen_test

@@ -15,6 +15,7 @@ init_tests \
 	abuild_keygen_kernel \
 
 export ABUILD_SHAREDIR="$SRCDIR"/..
+export ABUILD_USERDIR="$HOME"/.abuild
 export GIT=false
 export FAKEKEY="$SRCDIR/testdata/abuild.key"
 export FAKEKEYPUB="$SRCDIR/testdata/abuild.key.pub"

tests/abuild_sign_test

@@ -12,6 +12,7 @@ init_tests \
 	abuild_sign_owner
 
 export ABUILD_SHAREDIR="$(atf_get_srcdir)/.."
+export ABUILD_USERDIR="$HOME"/.abuild
 
 init_keys() {
 	cp -ra "$(atf_get_srcdir)"/testdata/.abuild "$PWD"