tests/perf: fix doc creation in basic-gets (#8797)

If test param ITERATIONS is specified as larger than 1000, after the 1000th
iteration this test currently changes behaviour from fetching existing docs to
fetching non-existent docs.

The original intention seems to be that the number of docs created should be the
same as the number of iterations.  See e.g. 7089d39

Author: alxndrsn

tests/performance/perf.basics.js

@@ -125,7 +125,7 @@ module.exports = function (PouchDB, callback) {
       iterations: 1000,
       setup: function (db, callback) {
         var docs = [];
-        for (var i = 0; i < 1000; i++) {
+        for (var i = 0; i < this.iterations; i++) {
           docs.push({_id : commonUtils.createDocId(i),
             foo : 'bar', baz : 'quux'});
         }