write a go solution for Description:
A binary string is a string where each character is either 0 or 1. Two binary strings a and b of equal length are similar, if they have the same character in some position (there exists an integer i such that a_i=b_i). For example:

- 10010 and 01111 are similar (they have the same character in position 4);
- 10010 and 11111 are similar;
- 111 and 111 are similar;
- 0110 and 1001 are not similar.

You are given an integer n and a binary string s consisting of 2n-1 characters. Let's denote s[l..r] as the contiguous substring of s starting with l-th character and ending with r-th character (in other words, s[l..r]=s_ls_l+1s_l+2...s_r).

You have to construct a binary string w of length n which is similar to all of the following strings: s[1..n], s[2..n+1], s[3..n+2], ..., s[n..2n-1].

Input Format:
The first line contains a single integer t (1<=t<=1000) — the number of test cases.

The first line of each test case contains a single integer n (1<=n<=50).

The second line of each test case contains the binary string s of length 2n-1. Each character s_i is either 0 or 1.

Output Format:
For each test case, print the corresponding binary string w of length n. If there are multiple such strings — print any of them. It can be shown that at least one string w meeting the constraints always exists.

Note:
The explanation of the sample case (equal characters in equal positions are bold):

The first test case:

- mathbf1 is similar to s[1..1]=mathbf1.

The second test case:

- mathbf000 is similar to s[1..3]=mathbf000;
- mathbf000 is similar to s[2..4]=mathbf000;
- mathbf000 is similar to s[3..5]=mathbf000.

The third test case:

- mathbf10mathbf10 is similar to s[1..4]=mathbf11mathbf10;
- mathbf101mathbf0 is similar to s[2..5]=mathbf110mathbf0;
- mathbf101mathbf0 is similar to s[3..6]=mathbf100mathbf0;
- 1mathbf01mathbf0 is similar to s[4..7]=0mathbf00mathbf0.

The fourth test case:

- 0mathbf0 is similar to s[1..2]=1mathbf0;
- mathbf00 is similar to s[2..3]=mathbf01.. Output only the code with no comments, explanation, or additional text.