backtester/engine: Fix TestStart race (#2024)

* Fix TestStart race

TestStart does 3 tests
test1 = Start() with no error
test2 = Start() on an already started struct
test3 = Start() on nil

Previously, test1 and test2 were on the same struct. Depending on
the go scheduler, we could have test2 fail because the field "started"
could go from 1 to 0 just after the atomic compare and swap.
So we would Start() a second time on the same struct but the field "started"
would already be back to 0.

* Divide TestStart in 3 tests

* Resolve comments

---------

Co-authored-by: mathieu cesbron <mathieucesbron@mathieus-MacBook-Pro.local>

backtester/engine/live_test.go

@@ -2,7 +2,6 @@ package engine
 
 import (
 	"sync"
-	"sync/atomic"
 	"testing"
 	"time"
 
@@ -65,21 +64,25 @@ func TestSetupLiveDataHandler(t *testing.T) {
 
 func TestStart(t *testing.T) {
 	t.Parallel()
-	dc := &dataChecker{
+
+	var dc *dataChecker
+	err := dc.Start()
+	assert.ErrorIs(t, err, gctcommon.ErrNilPointer)
+
+	dc = &dataChecker{
 		shutdown: make(chan bool),
 	}
-	err := dc.Start()
-	assert.NoError(t, err)
+	err = dc.Start()
+	require.NoError(t, err)
 
 	close(dc.shutdown)
 	dc.wg.Wait()
-	atomic.CompareAndSwapUint32(&dc.started, 0, 1)
+
+	dc = &dataChecker{
+		started: 1,
+	}
 	err = dc.Start()
 	assert.ErrorIs(t, err, engine.ErrSubSystemAlreadyStarted)
-
-	var dh *dataChecker
-	err = dh.Start()
-	assert.ErrorIs(t, err, gctcommon.ErrNilPointer)
 }
 
 func TestDataCheckerIsRunning(t *testing.T) {