add testcase for rangeForTimestamp. (#6454)

Signed-off-by: johncming <johncming@yahoo.com>

tsdb/db_test.go

@@ -2579,6 +2579,28 @@ func TestChunkWriter_ReadAfterWrite(t *testing.T) {
 	}
 }
 
+func TestRangeForTimestamp(t *testing.T) {
+	type args struct {
+		t     int64
+		width int64
+	}
+	tests := []struct {
+		args     args
+		expected int64
+	}{
+		{args{0, 5}, 5},
+		{args{1, 5}, 5},
+		{args{5, 5}, 10},
+		{args{6, 5}, 10},
+		{args{13, 5}, 15},
+		{args{95, 5}, 100},
+	}
+	for _, tt := range tests {
+		got := rangeForTimestamp(tt.args.t, tt.args.width)
+		testutil.Equals(t, tt.expected, got)
+	}
+}
+
 // TestChunkReader_ConcurrentReads checks that the chunk result can be read concurrently.
 // Regression test for https://github.com/prometheus/prometheus/pull/6514.
 func TestChunkReader_ConcurrentReads(t *testing.T) {