PT-193 Fixed regex in TableParser

TableParser's parse function was failing while trying to lowercase
column names in the provided 'SHOW CREATE TABLE'.
The problem was it was trying to lowercase everything between backticks
but lines like these:

`field_name` int comment "here is a ` in the comment"
`second_field_name` int

made the original regex to fail, matching `in the coment"` as an
expression to be lowercased while second_file_name was considered as
outside backticks.

t/lib/samples/issue_pt-193_backtick_in_col_comments.sql

@@ -0,0 +1,6 @@
+Create Table: CREATE TABLE `t3` (
+    `id` int(11) NOT NULL AUTO_INCREMENT,
+    `f22aBcDe` int(10) unsigned DEFAULT NULL COMMENT 'xxx`XXx',
+    `f23aBc` int(10) unsigned NOT NULL DEFAULT '255' COMMENT "`yyy",
+    PRIMARY KEY (`id`)
+) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=latin1)