test: vinyl/mvcc: check uniqueness constraint for secondary indexes

The check was accidentally broken by commit eb5cd536 ("vinyl: do not
track partial reads in tx manager"). Add a test case to avoid similar
screw-ups in future.

See #2716

test/vinyl/mvcc.result

@@ -3918,6 +3918,49 @@ c1:commit()
 ---
 - 
 ...
+t:truncate()
+---
+...
+--
+-- gh-2716 uniqueness check for secondary indexes
+--
+_ = t:create_index('sk', {parts = {2, 'unsigned'}, unique = true})
+---
+...
+c1:begin()
+---
+- 
+...
+c2:begin()
+---
+- 
+...
+c1("t:insert{1, 2}")
+---
+- - [1, 2]
+...
+c2("t:insert{2, 2}")
+---
+- - [2, 2]
+...
+c1:commit()
+---
+- 
+...
+c2:commit() -- rollback
+---
+- - {'error': 'Transaction has been aborted by conflict'}
+...
+t:select() -- {1, 2}
+---
+- - [1, 2]
+...
+t:truncate()
+---
+...
+t.index.sk:drop()
+---
+...
 -- *************************************************************************
 -- 1.7 cleanup marker: end of tests cleanup
 -- *************************************************************************

test/vinyl/mvcc.test.lua

@@ -1550,6 +1550,21 @@ c2("t:replace{2, 'new'}")
 c2:commit()
 c1("t:select()") -- {1, 'new'}, {2, 'new'}
 c1:commit()
+t:truncate()
+
+--
+-- gh-2716 uniqueness check for secondary indexes
+--
+_ = t:create_index('sk', {parts = {2, 'unsigned'}, unique = true})
+c1:begin()
+c2:begin()
+c1("t:insert{1, 2}")
+c2("t:insert{2, 2}")
+c1:commit()
+c2:commit() -- rollback
+t:select() -- {1, 2}
+t:truncate()
+t.index.sk:drop()
 
 -- *************************************************************************
 -- 1.7 cleanup marker: end of tests cleanup