Fix testing problem when 'current' equal to 'now' for separate inserts

 on fast machines with fast disks.
Adjust a few other tests to be more complete.

src/test/regress/expected/datetime.out

+QUERY: SELECT ('today'::datetime = ('yesterday'::datetime + '1 day'::timespan)) as "True";
+True
+----
+t   
+(1 row)
+
+QUERY: SELECT ('today'::datetime = ('tomorrow'::datetime - '1 day'::timespan)) as "True";
+True
+----
+t   
+(1 row)
+
+QUERY: SELECT ('tomorrow'::datetime = ('yesterday'::datetime + '2 days'::timespan)) as "True";
+True
+----
+t   
+(1 row)
+
+QUERY: SELECT ('current'::datetime = 'now'::datetime) as "True";
+True
+----
+t   
+(1 row)
+
+QUERY: SELECT ('now'::datetime - 'current'::datetime) AS "ZeroSecs";
+ZeroSecs
+--------
+@ 0     
+(1 row)
+
 QUERY: CREATE TABLE DATETIME_TBL( d1 datetime);
 QUERY: INSERT INTO DATETIME_TBL VALUES ('current');
-QUERY: INSERT INTO DATETIME_TBL VALUES ('now');
 QUERY: INSERT INTO DATETIME_TBL VALUES ('today');
 QUERY: INSERT INTO DATETIME_TBL VALUES ('yesterday');
 QUERY: INSERT INTO DATETIME_TBL VALUES ('tomorrow');
@@ -36,12 +65,6 @@ one
   1
 (1 row)
 
-QUERY: SELECT 'now'::datetime - 'current'::datetime AS ZeroSecs;
-zerosecs
---------
-@ 0     
-(1 row)
-
 QUERY: SELECT count(*) AS one FROM DATETIME_TBL WHERE d1 = 'now'::datetime;
 one
 ---

src/test/regress/sql/datetime.sql

 -- *** testing new built-in time types: datetime, timespan ***
 
-CREATE TABLE DATETIME_TBL( d1 datetime);
-
 -- Shorthand values
--- Not directly testable since these are not constant for regression testing.
+-- Not directly usable for regression testing since these are not constants.
 -- So, just try to test parser and hope for the best - tgl 97/04/26
+
+SELECT ('today'::datetime = ('yesterday'::datetime + '1 day'::timespan)) as "True";
+SELECT ('today'::datetime = ('tomorrow'::datetime - '1 day'::timespan)) as "True";
+SELECT ('tomorrow'::datetime = ('yesterday'::datetime + '2 days'::timespan)) as "True";
+SELECT ('current'::datetime = 'now'::datetime) as "True";
+SELECT ('now'::datetime - 'current'::datetime) AS "ZeroSecs";
+
+CREATE TABLE DATETIME_TBL( d1 datetime);
+
 INSERT INTO DATETIME_TBL VALUES ('current');
-INSERT INTO DATETIME_TBL VALUES ('now');
 INSERT INTO DATETIME_TBL VALUES ('today');
 INSERT INTO DATETIME_TBL VALUES ('yesterday');
 INSERT INTO DATETIME_TBL VALUES ('tomorrow');
@@ -19,8 +25,8 @@ SELECT count(*) AS one FROM DATETIME_TBL WHERE d1 = 'yesterday'::datetime;
 SELECT count(*) AS one FROM DATETIME_TBL WHERE d1 = 'today'::datetime + '1 day'::timespan;
 SELECT count(*) AS one FROM DATETIME_TBL WHERE d1 = 'today'::datetime - '1 day'::timespan;
 
-SELECT 'now'::datetime - 'current'::datetime AS ZeroSecs;
 SELECT count(*) AS one FROM DATETIME_TBL WHERE d1 = 'now'::datetime;
+
 DELETE FROM DATETIME_TBL;
 
 -- verify uniform transaction time within transaction block