sql: fix flaky collation test

Sometimes this test failed because collation 'c' was not dropped
automatically. Now it is always dropped manually, so the problem
is no more.

test/sql/collation.result

@@ -796,3 +796,6 @@ box.space._collation.index.name:get({'c'})
 ---
 - [4, 'c', 1, 'ICU', 'unicode', {'strength': 'tertiary'}]
 ...
+box.internal.collation.drop('c')
+---
+...

test/sql/collation.test.lua

@@ -313,3 +313,4 @@ box.space.JJ:drop()
 --
 box.internal.collation.create('c', 'ICU', 'unicode')
 box.space._collation.index.name:get({'c'})
+box.internal.collation.drop('c')