Add regression tests for KNN

[strk]

I could not find regression tests for ORDER BY <-> and <#>. If they are really missing, this ticket is to add them.

[robe]

lets make this 2.2

[strk]

With r13235 I've extracted existing operator tests into a new "operators.sql" file, that's where new tests should be hosted.

Beside, tests for other operators are missing, noted in r13236

[strk]

A couple of tests were added with r13244. Would probably be useful to add more for the <#> one

[strk]

r13259 in 2.1 branch (2.1.6)

[strk]

tests are always good in stable branches...

[strk]

tests that the KNN operator actually make use of an index have been committed as r13274 in 2.1 branch (2.1.6) and r13272 in trunk (2.2.0)