Improve testcase condition statement for dialects

Improve testcase condition statement for dialects

We could get dialect.requires_name_normalize rather than use hard code
as "firebird" or "oracle", since we have add `normalize` attribute for
quite a long time.

### Description

Use `dialect.requires_name_normalize` instead `testing.against("firebird", "oracle")`

### Checklist
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This pull request is:

- [ ] A documentation / typographical error fix
	- Good to go, no issue or tests are needed
- [x] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [ ] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

**Have a nice day!**

Closes: #4843
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4843
Pull-request-sha: 304fe67b06c1f010a164806a12b68cfce9bd0d2b

Change-Id: I276f781482779473258f9269074847e283711b05