Make coresub.t faster by eschewing B::Deparse

Just use B directly to look at the op tree, instead of checking that
deparsed op trees are identical.

Before:

real	0m4.021s
user	0m3.668s
sys	0m0.191s

After:

real	0m0.388s
user	0m0.251s
sys	0m0.080s
Suppress ‘-e syntax OK’ in concise.t

On Windows, the output was including

# 490-e syntax OK
# : 	    $l->concise($level);

which does not match /(?^:\# 4\d\d: \s+ \$l->concise\(\$level\);)/,
hence the test failure.