tests: add integration tests

to reproduce crash on teardown when using JACK as audio driver.

Test is not done yet. It works properly on non-zero exit codes but not on crashes (which is what we encounter here)

src/tests/README.md

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+Unit test of Hydrogen's core library. Not to be confused with our [integration tests](../../tests/).
+
+These tests are executed in our **AppVeyor** build pipeline as well as when calling
+our [build.sh](../build.sh) script with `t` as argument.

tests/README.md

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+Integration tests used to check more complex use-cases we can not cover using
+our [unit tests](../src/tests/).
+
+They are not routinely executed without our **AppVeyor** build pipeline or when
+calling our [build.sh](../build.sh) script with `t` as argument. But they are,
+nevertheless, designed to exit with code `0` on success and with `1` on failure.

tests/jackTearDown/README.md

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+Integration test to check whether Hydrogen does properly exit without crashing
+when using the JACK audio driver.
+
+## Requirements
+
+- `hydrogen` - the system-wide installation is used
+- `Go` >= 1.20
+- `JACK`
+
+The code is written and tested on **Linux** Devuan. But since the `OSC` package
+is written in pure `Go` and all other packages used are part of the standard
+library the code _should_ run on **macOS** and **Windows** as well.
+
+## Usage
+
+``` bash
+go run main.go
+```

tests/jackTearDown/go.mod

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+module hydrogen/integrationTests/jackTearDown
+
+go 1.20
+
+require github.com/hypebeast/go-osc v0.0.0-20220308234300-cec5a8a1e5f5

tests/jackTearDown/go.sum

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+github.com/hypebeast/go-osc v0.0.0-20220308234300-cec5a8a1e5f5 h1:fqwINudmUrvGCuw+e3tedZ2UJ0hklSw6t8UPomctKyQ=
+github.com/hypebeast/go-osc v0.0.0-20220308234300-cec5a8a1e5f5/go.mod h1:lqMjoCs0y0GoRRujSPZRBaGb4c5ER6TfkFKSClxkMbY=

tests/jackTearDown/main.go

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+package main
+
+import (
+    "fmt"
+    "log"
+    "os/exec"
+    "time"
+
+    "github.com/hypebeast/go-osc/osc"
+)
+
+// How many times the test should be repeated.
+const numberOfTestRuns = 100
+const oscHydrogenPort = 9000
+
+// hydrogenStartupTime gives an upper limit for the time Hydrogen requires to
+// start up in milliseconds.
+const hydrogenStartupTime = 3000
+
+// hydrogenStartupChan is used by startHydrogen() to indicate that Hydrogen was
+// started.
+var hydrogenStartupChan chan bool
+
+// Integration test checking for errors/crashes when closing Hydrogen while
+// using the JACK driver (on Linux).
+func main() {
+    var err error
+
+    _, err = exec.LookPath("hydrogen")
+    if err != nil {
+        log.Fatalf("[hydrogen] executable could not be found: %v", err.Error())
+    }
+
+    oscClient := osc.NewClient("localhost", oscHydrogenPort)
+
+    hydrogenStartupChan = make(chan bool, 1)
+
+    for ii := 0; ii < numberOfTestRuns; ii++ {
+
+        go killHydrogen(oscClient)
+
+        err = startHydrogen()
+
+        if err != nil {
+            log.Fatalf("Hydrogen exited with non-zero code: %v",
+                err.Error())
+        }
+        log.Printf("Instance [%v] terminated without error\n", ii)
+    }
+}
+
+// Start up Hydrogen using the JACK driver and with a configuration that both
+// enables OSC and uses a specific port unlikely used by another Hydrogen
+// instance (in case another one is already running).
+func startHydrogen() error {
+    hydrogenStartupChan <- true
+
+    cmd := exec.Command("hydrogen", "--driver", "jack", "--nosplash")
+    output, err := cmd.Output()
+    if err != nil {
+        return fmt.Errorf("[startHydrogen] Exited with error [%v]:\n%v\n\n",
+            err, string(output))
+    }
+
+    return nil
+}
+
+func killHydrogen(client *osc.Client) {
+    for {
+        select {
+        case <-hydrogenStartupChan:
+            // Give Hydrogen some time to start up properly
+            time.Sleep(hydrogenStartupTime * time.Millisecond)
+
+            // Hydrogen is ready. Let's shut it down.
+            msg := osc.NewMessage("/Hydrogen/QUIT")
+            err := client.Send(msg)
+            if err != nil {
+                log.Fatalf("[killHydrogen] Unable to send OSC message: %v",
+                    err.Error())
+            }
+            return
+
+        default:
+            time.Sleep(100 * time.Millisecond)
+            continue
+        }
+    }
+}