t = int(input())
 
for _ in range(t):
    n, m = map(int, input().split())
    grid = [input() for _ in range(n)]
 
    possible = True
 
    # Check horizontal pairs
    for i in range(n):
        for j in range(m - 1):
            if grid[i][j] != grid[i][j + 1]:
                possible = False
 
    # Check vertical pairs
    for j in range(m):
        for i in range(n - 1):
            if grid[i][j] != grid[i + 1][j]:
                possible = False
 
    if possible:
        print("YES")
    else:
        print("NO")
 

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