add tests for validate

* valid case
* invalid case

validate_test.go

+package gitlab
+
+import (
+	"fmt"
+	"net/http"
+	"reflect"
+	"testing"
+)
+
+// TODO: better test posting empty string
+func TestValidate(t *testing.T) {
+	validContent := `
+	build1:
+		stage: build
+		script:
+			- echo "Do your build here"`
+
+	invalidContent := `
+	build1:
+		- echo "Do your build here"`
+
+	validRes := `{
+			"status": "valid",
+			"errors": []
+		}`
+
+	invalidRes := `{
+			"status": "invalid",
+			"errors": [
+				"error message when content is invalid"
+			]
+		}`
+
+	wantValid := &LintResult{
+		Status: "valid",
+		Errors: []string{},
+	}
+
+	wantInvalid := &LintResult{
+		Status: "invalid",
+		Errors: []string{"error message when content is invalid"},
+	}
+
+	testCases := []struct {
+		desc    string
+		content string
+		res     string
+		want    *LintResult
+	}{
+		{"valid", validContent, validRes, wantValid},
+		{"invalid", invalidContent, invalidRes, wantInvalid},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.desc, func(t *testing.T) {
+			mux, server, client := setup()
+			defer teardown(server)
+
+			mux.HandleFunc("/ci/lint", func(w http.ResponseWriter, r *http.Request) {
+				testMethod(t, r, "POST")
+				fmt.Fprintf(w, tc.res)
+			})
+
+			got, _, err := client.Validate.Lint(tc.content)
+
+			if err != nil {
+				t.Errorf("Validate returned error: %v", err)
+			}
+
+			want := tc.want
+			if !reflect.DeepEqual(got, want) {
+				t.Errorf("Validate returned \ngot:\n%v\nwant:\n%v", Stringify(got), Stringify(want))
+			}
+		})
+	}
+}