add unit tests (#60)

also, flatten the workspace into a single crate. I forget what the original reason for the workspace
was -- I think it was added because `cargo t -p testsuite` is nicer than running
`cargo t --test $filename` N times. However, a single crate removes the need to repeat the list of
dependencies in `testsuite/Cargo.toml`

also, document the two sets of tests included with the template and how to run them in the README
file

tests/integration.rs

@@ -0,0 +1,16 @@
+#![no_std]
+#![no_main]
+
+use {{crate_name}} as _; // memory layout + panic handler
+
+// See https://crates.io/crates/defmt-test/0.3.0 for more documentation (e.g. about the 'state'
+// feature)
+#[defmt_test::tests]
+mod tests {
+    use defmt::assert;
+
+    #[test]
+    fn it_works() {
+        assert!(true)
+    }
+}