test: Fix bug in TestAddTestPlugin test helper (hashicorp#19313)

* fix external plugin test failing locally

* Ensure file is closed and written in TestAddTestPlugin

vault/external_tests/plugin/plugin_test.go

@@ -423,6 +423,8 @@ func testPlugin_continueOnError(t *testing.T, btype logical.BackendType, mismatc
 
 			// Get the registered plugin
 			req := logical.TestRequest(t, logical.ReadOperation, fmt.Sprintf("sys/plugins/catalog/%s/mock-plugin", pluginType))
+			// We are using the mock backend from vault/sdk/plugin/mock/backend.go which sets the plugin version.
+			req.Data["version"] = "v0.0.0+mock"
 			req.ClientToken = core.Client.Token()
 			resp, err := core.HandleRequest(namespace.RootContext(testCtx), req)
 			if err != nil || resp == nil || (resp != nil && resp.IsError()) {

vault/testing.go

@@ -539,6 +539,9 @@ func TestAddTestPlugin(t testing.T, c *Core, name string, pluginType consts.Plug
 		if err != nil {
 			t.Fatal(err)
 		}
+		// Ensure that the file is closed and written. This seems to be
+		// necessary on Linux systems.
+		out.Close()
 
 		dirPath = tempDir
 	}