Testing extending a list
Use extend() instead of append() to add a list to another list, and use the fact that None and empty lists compare as False, while a non-empty list compares as True, and omit the redundant return statement:
def test(x1, x2):
    if x2:
        x1.extend(x2)

As for reducing the number of print statements, prepare a list or tuple with your test cases, and handle them all in a single for-loop:
l = ['tt', 'yy', 'pp']
testcases = [
    None,
    ['hello'],
    ['hello', 'world']
]

for testcase in testcases:
    test(l, testcase)
    print(l)