Add t/re/anyof.t

Add t/re/anyof.t

The ANYOF regnode is by far the most complicated to compile in regular
expression patterns.  This new test file makes sure that what gets
compiled doesn't get changed accidentally, nor how the compiled node is
dumped for human readable output.

I created tests for this from what changed in
b77aba85f6ec3b2a2341077b14f39261c5753cea
"Revamp -Dr handling of /[...]/", and from trying to exercise all
branches in it using gcov.

Making this test file showed some bugs, and infelicitous representations
from that commit, which will be fixed in the next few commits, and tests
added as each change is done.