tests: Run `download-using-manifests' via libtool so that dlopening works.

substitute.mk

@@ -1,5 +1,7 @@
 %: %.in Makefile
 	sed \
+	 -e "s^@abs_top_srcdir\@^$(abs_top_srcdir)^g" \
+	 -e "s^@abs_top_builddir\@^$(abs_top_builddir)^g" \
 	 -e "s^@extra1\@^$(extra1)^g" \
 	 -e "s^@prefix\@^$(prefix)^g" \
 	 -e "s^@bindir\@^$(bindir)^g" \

tests/common.sh.in

@@ -1,5 +1,8 @@
 set -e
 
+export top_srcdir="@abs_top_srcdir@"
+export top_builddir="@abs_top_builddir@"
+
 export TOP=$(pwd)/..
 
 export TEST_ROOT=$(pwd)/test-tmp

tests/init.sh

@@ -47,7 +47,9 @@ EOF
 cat > $NIX_BIN_DIR/nix/substituters/download-using-manifests.pl <<EOF
 #! $SHELL -e
 export DYLD_LIBRARY_PATH=
-exec $TOP/scripts/download-using-manifests.pl "\$@"
+exec "$top_builddir/libtool" --mode=execute			\
+  -dlopen "$top_builddir/perl/libNixStore.la"			\
+  "$top_builddir/scripts/download-using-manifests.pl" "\$@"
 EOF
 chmod +x $NIX_BIN_DIR/nix/substituters/download-using-manifests.pl