Replace flaky server for log level handler test. (#9822)

**What this PR does / why we need it**:
I've noticed that the `TestLevelHandler` were flaky because the code
wasn't waiting to the server to be running. Since the test is for a
handler it's much simpler to call it directly with a response recorder.

**Checklist**
- [x] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [ ] Documentation added
- [x] Tests updated
- [ ] `CHANGELOG.md` updated
- [ ] If the change is worth mentioning in the release notes, add
`add-to-release-notes` label
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/upgrading/_index.md`
- [ ] For Helm chart changes bump the Helm chart version in
`production/helm/loki/Chart.yaml` and update
`production/helm/loki/CHANGELOG.md` and
`production/helm/loki/README.md`. [Example
PR](d10549e3ec)

pkg/util/log/log_test.go

@@ -3,6 +3,7 @@ package log
 import (
 	"io"
 	"net/http"
+	"net/http/httptest"
 	"net/url"
 	"strings"
 	"testing"
@@ -20,12 +21,6 @@ func TestLevelHandler(t *testing.T) {
 		baseLogger: log.NewLogfmtLogger(io.Discard),
 	}
 
-	// Start test http server
-	go func() {
-		err := http.ListenAndServe(":8080", LevelHandler(&lvl))
-		assert.NoError(t, err)
-	}()
-
 	testCases := []struct {
 		testName           string
 		targetLogLevel     string
@@ -42,22 +37,25 @@ func TestLevelHandler(t *testing.T) {
 	for _, testCase := range testCases {
 		t.Run(testCase.testName, func(t *testing.T) {
 			var (
-				resp *http.Response
-				err  error
+				req *http.Request
+				err error
 			)
 
 			if strings.HasPrefix(testCase.testName, "Get") {
-				resp, err = http.Get("http://localhost:8080/")
+				req, err = http.NewRequest("GET", "/", nil)
 			} else if strings.HasPrefix(testCase.testName, "Post") {
-				resp, err = http.PostForm("http://localhost:8080/", url.Values{"log_level": {testCase.targetLogLevel}})
+				form := url.Values{"log_level": {testCase.targetLogLevel}}
+				req, err = http.NewRequest("POST", "/", strings.NewReader(form.Encode()))
+				req.Header.Add("Content-Type", "application/x-www-form-urlencoded")
 			}
 			assert.NoError(t, err)
-			defer resp.Body.Close()
 
-			body, err := io.ReadAll(resp.Body)
-			assert.NoError(t, err)
-			assert.JSONEq(t, testCase.expectedResponse, string(body))
-			assert.Equal(t, testCase.expectedStatusCode, resp.StatusCode)
+			rr := httptest.NewRecorder()
+			handler := LevelHandler(&lvl)
+			handler.ServeHTTP(rr, req)
+
+			assert.JSONEq(t, testCase.expectedResponse, rr.Body.String())
+			assert.Equal(t, testCase.expectedStatusCode, rr.Code)
 			assert.Equal(t, testCase.expectedLogLevel, lvl.String())
 		})
 	}