[BUGS-5759] Adjust logic for checking for options table

[jspellman814]

Addresses #136 where customers use a custom DB prefix without a trailing _.

[pwtyler]

If we're specifically looking for the wp_options table, (as opposed to say, wp_mycustompluginoptions), can we use the global $table_prefix + options instead of a preg_match?

[jspellman814]

I was able to test this change successfully on my local machine via lando wp. Steps taken:

1. Start lando container on local site: lando start
2. Install wp_launch_check package: lando wp package install pantheon-systems/wp_launch_check
3. Run lando wp launchcheck database to get baseline test.
4. SSH into container: lando ssh
5. cd to directory containing the file that needs to be edited: cd /var/www/.wp-cli/packages/vendor/pantheon-systems/wp_launch_check/php/pantheon/checks/
6. Remove current database.php file and create new one with contents from this PR: rm database.php, cat > database.php and then paste the contents of the file from this branch.
7. Confirm update by running lando wp launchcheck database

Database check prior to change:

Database check after change (note the new conditional!! text):

I also changed the table prefix and am seeing the same results. Going to test this on the platform as per CONTRIBUTING.md. Assuming that goes well, this will be merged and deployed next week.