Stabilize regression test result.

If random() returns a result sufficiently close to zero, float8out
switches to scientific notation, breaking this test case's expectation
that the output should look like '0.xxxxxxxxx'.  Casting to numeric
should fix that.  Per buildfarm member pogona.

Discussion: https://postgr.es/m/20180324212502.wt4serghfidge2on@alap3.anarazel.de

src/test/regress/expected/create_procedure.out

@@ -22,7 +22,7 @@ LINE 1: SELECT ptest1('x');
 HINT:  To call a procedure, use CALL.
 CALL ptest1('a');  -- ok
 CALL ptest1('xy' || 'zzy');  -- ok, constant-folded arg
-CALL ptest1(substring(random()::text, 1, 1));  -- ok, volatile arg
+CALL ptest1(substring(random()::numeric(20,15)::text, 1, 1));  -- ok, volatile arg
 \df ptest1
                         List of functions
  Schema |  Name  | Result data type | Argument data types | Type 

src/test/regress/sql/create_procedure.sql

@@ -14,7 +14,7 @@ $$;
 SELECT ptest1('x');  -- error
 CALL ptest1('a');  -- ok
 CALL ptest1('xy' || 'zzy');  -- ok, constant-folded arg
-CALL ptest1(substring(random()::text, 1, 1));  -- ok, volatile arg
+CALL ptest1(substring(random()::numeric(20,15)::text, 1, 1));  -- ok, volatile arg
 
 \df ptest1
 SELECT pg_get_functiondef('ptest1'::regproc);