HOTFIX: clear() configs in SQLConf-related unit tests.

Thanks goes to @liancheng, who pointed out that `sql/test-only *.SQLConfSuite *.SQLQuerySuite` passed but `sql/test-only *.SQLQuerySuite *.SQLConfSuite` failed. The reason is that some tests use the same test keys and without clear()'ing, they get carried over to other tests. This hotfix simply adds some `clear()` calls.

This problem was not evident on Jenkins before probably because `parallelExecution` is not set to `false` for `sqlCoreSettings`.

Author: Zongheng Yang <zongheng.y@gmail.com>

Closes #1040 from concretevitamin/sqlconf-tests and squashes the following commits:

6d14ceb [Zongheng Yang] HOTFIX: clear() confs in SQLConf related unit tests.

(cherry picked from commit 601032f5bfe2dcdc240bfcc553f401e6facbf5ec)
Signed-off-by: Michael Armbrust <michael@databricks.com>