t/testc.sh: add 301 and 324

need new t/issue301.t
with 301 package B is not compiled in. special case for B I guess.
324 misses Master and Slave though.
C: detect ->next::method (and more)

check for special next|maybe|NEXT::EVERY methods and include the package
if used. Fixes issues 301 and 324.
threaded const->PVs are especially hard to access
MANIFEST: add new test