Improvements to test_computed_col_default_not_set

Improvements to test_computed_col_default_not_set

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### Description
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- Update the test so it looks at computed_default_table instead of
  computed_column_table.
- Stop inspecting autoincrement; instead directly check for
  default/non-default values for the relevant columns.

Fixes: #5414
### 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: #5417
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5417
Pull-request-sha: 5918af6abb991e6d2322ea4252e56d456da628dc

Change-Id: Id9484ae7f297735345016be5b7461325825503da