test: remove redundant os.Exit in TestMain (#5257)

This PR simplifies tests by removing redundant `os.Exit` calls in
`TestMain` functions.

As of Go 1.15, we do not need to call `os.Exit(m.Run())` explicitly, as
value returned by `m.Run()` is stored into unexported field of `m` and
go test executable is smart enough to automatically call
`os.Exit(retValue)` when `TestMain` returns. See golang/go#34129.

internal/pipe/blob/blob_minio_test.go

@@ -30,7 +30,8 @@ var listen string
 
 func TestMain(m *testing.M) {
 	if !testlib.InPath("docker") {
-		os.Exit(m.Run())
+		m.Run()
+		return
 	}
 	prepareEnv()
 
@@ -67,10 +68,9 @@ func TestMain(m *testing.M) {
 	}))
 	listen = "localhost:" + resource.GetPort("9000/tcp")
 
-	code := m.Run()
+	m.Run()
 
 	requireNoErr(pool.Purge(resource))
-	os.Exit(code)
 }
 
 func TestMinioUpload(t *testing.T) {