Use driver for RegexpParser tests

[abetomo]

Which issue(s) this PR fixes:

( #4066 (comment) )

What this PR does / why we need it:

I didn't know why it was commented out, but it looked good on Fluent::Test::Driver::Parser.new(...).
#1286

Docs Changes:

None

Release Note:

None

[daipom]

Thanks! We should fix tests like this.

Certainly, the intent of the #1286 fix is unclear.

Fluent::Test::Driver::Parser::configure() calls instance.configure(), so I think the old code is meaningless.

I have confirmed the test succeeds even on #1286 with this fix.

It looks good to me.

[ashie]

Thanks!

[abetomo]

Thanks for the merge!