test: made each issue a subtest

Allow each test to be a subtest, so users can run things like:

```sh
go test -v ./... -run 'TestRendererIssues/107'
```

In order to run a single issue test, which helps when trying to fix a
new one.

Signed-off-by: Carlos A Becker <caarlos0@gmail.com>

ansi/renderer_test.go

@@ -105,6 +105,7 @@ func TestRendererIssues(t *testing.T) {
 
 	for _, f := range files {
 		bn := strings.TrimSuffix(filepath.Base(f), ".md")
+		t.Run(bn, func(t *testing.T) {
 			tn := filepath.Join(issuesDir, bn+".test")
 
 			in, err := ioutil.ReadFile(f)
@@ -153,7 +154,7 @@ func TestRendererIssues(t *testing.T) {
 				if err != nil {
 					t.Fatal(err)
 				}
-			continue
+				return
 			}
 
 			// verify
@@ -166,5 +167,6 @@ func TestRendererIssues(t *testing.T) {
 				t.Errorf("Rendered output for %s doesn't match!\nExpected: `\n%s`\nGot: `\n%s`\n",
 					bn, string(td), buf.String())
 			}
+		})
 	}
 }