Add tests for more row patterns with COPY FROM .. (ON_ERROR ignore)

While digging into the code of this feature, I got confused by the fact
that a line is skipped when a value cannot be converted to its expected
attribute even if the line has fewer attributes than the target
relation.  The tests had a check for the case of an empty line, this
commit a couple more patterns where a line is incomplete, but skipped
because of a conversion error.

Reviewed-by: Bharath Rupireddy
Discussion: https://postgr.es/m/Ze_7kZqexdt0BiyC@paquier.xyz

src/test/regress/expected/copy2.out

@@ -730,13 +730,14 @@ COPY check_ign_err FROM STDIN WITH (on_error stop);
 ERROR:  invalid input syntax for type integer: "a"
 CONTEXT:  COPY check_ign_err, line 2, column n: "a"
 COPY check_ign_err FROM STDIN WITH (on_error ignore);
-NOTICE:  4 rows were skipped due to data type incompatibility
+NOTICE:  6 rows were skipped due to data type incompatibility
 SELECT * FROM check_ign_err;
  n |  m  | k 
 ---+-----+---
  1 | {1} | 1
  5 | {5} | 5
-(2 rows)
+ 8 | {8} | 8
+(3 rows)
 
 -- test datatype error that can't be handled as soft: should fail
 CREATE TABLE hard_err(foo widget);

src/test/regress/sql/copy2.sql

@@ -515,6 +515,9 @@ a	{2}	2
 4	{a, 4}	4
 
 5	{5}	5
+6	a
+7	{7}	a
+8	{8}	8
 \.
 SELECT * FROM check_ign_err;