Longest absolute path in file system
The problem statement indicates that path separators should be taken into account when computing the length.  You might add a test case to test for this case either by crafting a particular input or by returning the longest path along with longest count that you can verify.

Some of the code could be made more concise.

filename = path.split('\t')[-1]    # last of list, assumes no tabs in filenames
depth = len(path) - len(filename)  # yields number of tabs


Rather than repeatedly computing the sum of the paths, you could create a cumulative_sum.

cumulative_len = cumulative_len[:depth]
if depth &gt; 0:
    # last element contains sum of entire path to depth
    # +1 to account for the preceding path separator
    cumulative_len.append(cumulative_len[-1] + len(filename) + 1)
else:
    # the problem statement does not include an initial path separator
    cumulative_len.append(len(filename))


Rather than joining the path for intermediate maximums that will be discarded, just store the list of path elements and join on final delivery.

You might consider asserting on the desired results so you get clearer feedback of failure.

def check(input, expected):
    longest_path = find_longest_path(input)
    assert input == expected, "Expected {} but got {}".format(expected, longest_path)
    print(longest_path)

if __name__ == "__main__":
    check("dir\n\tsubdir1\n\t\tfile1.ext\n\t\tsubsubdir1\n\tsubdir2\n\t\tsubsubdir2\n\t\t\tfile2.ext", 
          "dir/subdir2/subsubdir2/file2.ext")

    check("dir\n\tsubdir1\n\tsubdir2\n\t\tfile.ext",
          "dir/subdir2/file.ext")