tests: get current directory the perl way

instead of hoping to have it in the environment. This at
least broke tests on solaris.

Signed-off-by: Sven Nierlein <Sven.Nierlein@consol.de>
tests: negate test fails when not run from plugins folder

Signed-off-by: Sven Nierlein <Sven.Nierlein@consol.de>
tests: skip extented snmp tests if snmpd has no perl support

solaris snmpd has no perl support by default, so this test will fail.

Signed-off-by: Sven Nierlein <Sven.Nierlein@consol.de>