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

src/lib.rs

@@ -1,3 +1,4 @@
+#![no_main]
 #![no_std]
 
 use defmt_rtt as _; // global logger
@@ -20,3 +21,17 @@ pub fn exit() -> ! {
         cortex_m::asm::bkpt();
     }
 }
+
+// defmt-test 0.3.0 has the limitation that this `#[tests]` attribute can only be used
+// once within a crate. the module can be in any file but there can only be at most
+// one `#[tests]` module in this library crate
+#[cfg(test)]
+#[defmt_test::tests]
+mod unit_tests {
+    use defmt::assert;
+
+    #[test]
+    fn it_works() {
+        assert!(true)
+    }
+}