Create cuda context once for all cuda tests

All cuda tests succeed, even with go version 1.10 or higher.
This commit solves issue mumax#224

cuda/buffer_test.go

@@ -4,7 +4,6 @@ import "testing"
 
 // In case of memory leak, this will crash
 func TestBuffer(t *testing.T) {
-	Init(0)
 	m1 := [3]int{2, 1024, 2048}
 	m2 := [3]int{4, 1024, 2048}
 	a := Buffer(3, m1)
@@ -24,7 +23,6 @@ func TestBuffer(t *testing.T) {
 }
 
 func BenchmarkBuffer(b *testing.B) {
-	Init(0)
 	b.StopTimer()
 	m := [3]int{2, 1024, 2048}
 	a := Buffer(3, m)

cuda/init_test.go

@@ -0,0 +1,12 @@
+package cuda
+
+import (
+	"github.com/mumax/3/cuda/cu"
+)
+
+// needed for all other tests.
+func init() {
+	cu.Init(0)
+	ctx := cu.CtxCreate(cu.CTX_SCHED_AUTO, 0)
+	cu.CtxSetCurrent(ctx)
+}

cuda/reduce_test.go

@@ -12,7 +12,6 @@ import (
 var in1, in2, in3 *data.Slice
 
 func initTest() {
-	Init(0)
 	if in1 != nil {
 		return
 	}

cuda/slice_test.go

@@ -7,7 +7,6 @@ import (
 )
 
 func TestSlice(t *testing.T) {
-	Init(0)
 	N0, N1, N2 := 2, 4, 8
 	m := [3]int{N0, N1, N2}
 	N := N0 * N1 * N2
@@ -42,7 +41,6 @@ func TestSlice(t *testing.T) {
 }
 
 func TestCpy(t *testing.T) {
-	Init(0)
 	N0, N1, N2 := 2, 4, 32
 	N := N0 * N1 * N2
 	mesh := [3]int{N0, N1, N2}