[SPARK-12401][SQL] Add integration tests for postgres enum types

[maropu]

We can handle posgresql-specific enum types as strings in jdbc.
So, we should just add tests and close the corresponding JIRA ticket.

[SparkQA]

Test build #48755 has finished for PR 10596 at commit 6460aca.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[maropu]

@liancheng @yhuai ping

[SparkQA]

Test build #49386 has finished for PR 10596 at commit a24c798.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[maropu]

@liancheng @yhuai ping

[liancheng]

Sorry for being late. This LGTM, could you please rebase it? Then I can merge it. Thanks!

[maropu]

@liancheng Rebased. We need to wait for the test done.

[SparkQA]

Test build #50062 has finished for PR 10596 at commit dbc6829.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[maropu]

@liancheng Okay and ready to merge.

[maropu]

@liancheng ping

[liancheng]

Thank, I'm merging this!