Generate Unit Test for the following code using PyTest:
```py
class UnionFind(object):
    def __init__(self):
        self.set = []

    def get_id(self):
        self.set.append(len(self.set))
        return len(self.set)-1

    def find_set(self, x):
        if self.set[x] != x:
            self.set[x] = self.find_set(self.set[x])  # path compression.
        return self.set[x]

    def union_set(self, x, y):
        x_root, y_root = map(self.find_set, (x, y))
        if x_root != y_root:
            self.set[min(x_root, y_root)] = max(x_root, y_root)
``` The letter b should appear at least 6 times.
user