Test 10 String concatenation even faster method

added even faster string concatenation test:test10/t1.py, incrementing older t1, t2 to t2, t3
following b(): (just for test purpose, it is also named a() in code
is much faster than the old t1.py, so i incremented the old tn.py and added new t1.py as being the fastest:

def b():
    r=''
    for i in range(10):
        r='%s%d' %(r,i)
    return r

In [27]: timeit(a, number=1000000)
Out[27]: 3.4888150691986084

In [28]: timeit(b, number=1000000)
Out[28]: 2.244536876678467