fix get_table_offline for percpu hash table

Use get_value() instead of lookup() as the value as in the case of
percpu tables the ValueType is an std::vector that has to be resized
before performing the lookup function.

Add also some testing for it.

Solves https://github.com/iovisor/bcc/issues/1860



Signed-off-by: Mauricio Vasquez B <mauricio.vasquez@polito.it>