Add comment on test.

The commit message of commit a7412321 explained the reason well, but
let's also add it in a comment in the test itself.

src/test/regress/expected/column_compression.out

@@ -2,6 +2,8 @@
 -- Basic syntax
 -- Expect: success
 -----------------------------------------------------------------------
+-- These tests ALTER the default encoding options of some built-in types.
+-- Run them in a separate database to avoid messing with other tests.
 create database column_compression;
 \c column_compression
 prepare ccddlcheck as

src/test/regress/sql/column_compression.sql

@@ -3,6 +3,8 @@
 -- Expect: success
 -----------------------------------------------------------------------
 
+-- These tests ALTER the default encoding options of some built-in types.
+-- Run them in a separate database to avoid messing with other tests.
 create database column_compression;
 \c column_compression