Fix broken test_icmp due to locale

The float returned by tshark is locale-dependent.
Depending on the locale, a comma can be used instead of a dot as decimal
separator.

Test was failing on my machine due to the locale (fr_FR.UTF-8):
FAIL: test_icmp (__main__.TestFileCaptures)
Test to make sure ICMP fields are being read properly by comparing
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Traceback (most recent call last):
  File "pyshark/test/validate.py", line 48, in test_icmp
    self.assertEqual(resptime, '1.667')
AssertionError: '1,667' != '1.667'