Fix infinite loop in fs.md5sum / fs.sha1sum on Python 3

[potato-20]

The bug

reversec.common.fs.md5sum() and sha1sum() open the file in binary mode but drive their read loop with:

line = data = f.read()
while line != "":      # bytes compared to a str literal
    line = f.read()
    data += line

In Python 3, f.read() on a binary handle returns bytes, and b"" != "" is always True — so the loop never terminates. Any call on a readable file spins forever.

This is reachable in normal use: drozer/modules/common/superuser.py calls fs.md5sum() when verifying the bundled minimal-su binary, so that path hangs.

Reproduction (hangs before this fix):

from reversec.common import fs
fs.md5sum("/etc/hostname")   # never returns

The fix

Replace the hand-rolled loop with a single with open(...) read — the same pattern already used by fs.read() a few lines above:

def md5sum(path):
    try:
        with open(path, "rb") as f:
            return hashlib.md5(f.read()).hexdigest()
    except IOError:
        return None

sha1sum() gets the identical treatment, and its copy-pasted docstring (which said "md5sum") is corrected.

Tests

Adds regression tests in tests/test_fs.py asserting the digests match hashlib for normal and empty files, and that a missing file returns None. Full suite: 38 passed, no hang.

Note: this is stacked on #499 (which introduces the tests/ suite). Once #499 merges, this diff reduces to just the fs.py fix and its new tests.