Update decimal_casts.xml

tests/performance/decimal_casts.xml

 <test>
     <settings>
-        <max_memory_usage>10G</max_memory_usage>
+        <max_memory_usage>15G</max_memory_usage>
     </settings>
 
     <create_query>CREATE TABLE t (x UInt64, d32 Decimal32(3), d64 Decimal64(4), d128 Decimal128(5)) ENGINE = Memory</create_query>
     <!-- use less threads to save memory -->
-    <fill_query>INSERT INTO t SELECT number AS x, x % 1000000 AS d32, x AS d64, x d128 FROM numbers_mt(50000000) SETTINGS max_threads = 8</fill_query>
+    <fill_query>INSERT INTO t SELECT number AS x, x % 1000000 AS d32, x AS d64, x d128 FROM numbers_mt(100000000) SETTINGS max_threads = 8</fill_query>
     <drop_query>DROP TABLE IF EXISTS t</drop_query>
 
     <query>SELECT toUInt32(x)  y, toDecimal32(y, 1), toDecimal64(y, 5), toDecimal128(y, 6) FROM t FORMAT Null</query>