fstests: add TestInternal (#2085)

TestInternal allows to perform a custom test on the backend using the
optional InternalTester interface.

fstest/fstests/fstests.go

@@ -57,6 +57,13 @@ var (
 	isLocalRemote bool
 )
 
+// InternalTester is an optional interface for Fs which allows to execute internal tests
+//
+// This interface should be implemented in 'backend'_internal_test.go and not in 'backend'.go
+type InternalTester interface {
+	InternalTest(*testing.T)
+}
+
 // ExtraConfigItem describes a config item added on the fly while testing
 type ExtraConfigItem struct{ Name, Key, Value string }
 
@@ -970,6 +977,16 @@ func TestObjectPurge(t *testing.T) {
 	assert.Error(t, err, "Expecting error after on second purge")
 }
 
+// TestInternal calls InternalTest() on the Fs
+func TestInternal(t *testing.T) {
+	skipIfNotOk(t)
+	if it, ok := remote.(InternalTester); ok {
+		it.InternalTest(t)
+	} else {
+		t.Skipf("%T does not implement InternalTester", remote)
+	}
+}
+
 // TestFinalise tidies up after the previous tests
 func TestFinalise(t *testing.T) {
 	skipIfNotOk(t)