Add integration tests

Starting with C++ binaries. This work leverages the nix build system to
build the test programs [0] and executes them while running the profiler
to ensure the traces are what we expect.

[0]: https://github.com/javierhonduco/lightswitch-testprogs

tests/integration_test.rs

@@ -1,4 +1,4 @@
-use std::process::{Command, Child};
+use std::process::{Command, Child, Stdio};
 use std::time::Duration;
 
 use lightswitch::collector::Collector;
@@ -40,8 +40,8 @@ impl TestProcess {
     fn new(target: &str) -> Self{
         Self {
             child: Command::new(format!("./target/nix/bin/{}", target))
-            .stdout(std::process::Stdio::null())
-            .stderr(std::process::Stdio::null())
+            .stdout(Stdio::null())
+            .stderr(Stdio::null())
             .spawn()
             .unwrap()
         }
@@ -58,6 +58,7 @@ impl Drop for TestProcess {
     }
 }
 
+
 fn assert_any_stack_contains(
     symbolized_profile: &SymbolizedAggregatedProfile,
     expected_stack: &[&str],