Allow error message to be a suffix

[kentfredric]

Previously, the error message emitted by the native calls was of the form:

'syntax error'

However, more recently the error message takes the form:

'syntax error in filter expression: syntax error'

This patch pretends the part before the ":" is functionally equivalent to a line start, for the purposes of testing.

Closes: https://github.com/maddingue/Net-Pcap/issues/8

[maddingue]

Thanks, applied.