build-sys: Add a rust-test target

I often am editing just the Rust code, and want the fast iteration
feedback of a `cargo test` - don't want to pay the cost of full
optimization (particularly LTO) for the release build, and I
*just* want to run the Rust tests.

Basically if you're editing our Rust code a lot, this target is
your friend.

Closes: #1585
Approved by: jlebon

Makefile-rpm-ostree.am

@@ -129,3 +129,10 @@ rustfmt:
 	rustfmt $(LIBRPMOSTREE_RUST_SRCS)
 # Outside the ifdef, otherwise automake complains
 .PHONY: rustfmt
+
+# Wraps `cargo test`.  This is always a debug non-release build;
+# the main thing here is we still drop the `target` dir in our build
+# directory, since we nominally support srcdir != builddir.
+rust-test:
+	cd $(top_srcdir)/rust && CARGO_TARGET_DIR=@abs_top_builddir@/target cargo test
+.PHONY: rust-test