Enable a bunch of tests for std.net.isemail.

These tests were previously commented out because of a bug
the regular expression module. Now when we have a new regular
expression module we can enable these tests.

This also fixed a bug in the internally used "substr" function.
Use escape sequence for DELETE character.

This is just to make it a bit clearer what's going on.
Merge pull request #1217 from jacob-carlborg/isemail_unittests

Enable a bunch of tests for std.net.isemail.