t/t7006: ignore return status of shell's unset builtin

t/t7006: ignore return status of shell's unset builtin

The unset builtin of Solaris's xpg4/sh returns non-zero if it is passed a
variable name which was not previously set.  Since the unset is not likely
to fail, ignore its return status, but add a semicolon as a clue that the
'&&' was deliberately left off.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Acked-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>