Introduce one new alternative text output to fix flakiness (#5913)

Here is a flaky test output that is quite hard to fix:

```diff
diff -dU10 -w /home/circleci/project/src/test/regress/expected/isolation_master_update_node_1.out /home/circleci/project/src/test/regress/results/isolation_master_update_node.out
--- /home/circleci/project/src/test/regress/expected/isolation_master_update_node_1.out.modified	2022-03-21 19:03:54.237042562 +0000
+++ /home/circleci/project/src/test/regress/results/isolation_master_update_node.out.modified	2022-03-21 19:03:54.257043084 +0000
@@ -49,18 +49,20 @@
  <waiting ...>
 step s2-update-node-1-force: <... completed>
 master_update_node
 ------------------

 (1 row)

 step s2-abort: ABORT;
 step s1-abort: ABORT;
 FATAL:  terminating connection due to administrator command
-SSL connection has been closed unexpectedly
+server closed the connection unexpectedly
+	This probably means the server terminated abnormally
+	before or while processing the request.
```

I could not come up with a solution that would decrease the flakiness in the test outputs. We already have 3 output files for the same test and now I introduced a 4th one.

I can also add complex regular expressions that span multiple lines, and normalize these error messages. Feel free to suggest a normalized error message in a comment here.

## Current alternative file contents

`isolation_master_update_node.out`
```
step s1-abort: ABORT;
FATAL:  terminating connection due to administrator command
FATAL:  terminating connection due to administrator command
SSL connection has been closed unexpectedly
```

`isolation_master_update_node_0.out`
```
step s1-abort: ABORT;
WARNING: this step had a leftover error message
FATAL:  terminating connection due to administrator command
server closed the connection unexpectedly
	This probably means the server terminated abnormally
	before or while processing the request.
```

`isolation_master_update_node_1.out`
```
step s1-abort: ABORT;
FATAL:  terminating connection due to administrator command
SSL connection has been closed unexpectedly
```

new file: `isolation_master_update_node_2.out`
```
step s1-abort: ABORT;
FATAL:  terminating connection due to administrator command
server closed the connection unexpectedly
	This probably means the server terminated abnormally
	before or while processing the request.
```

(cherry picked from commit 518fb0873e)

src/test/regress/expected/isolation_master_update_node_2.out

@@ -0,0 +1,68 @@
+Parsed test spec with 2 sessions
+
+starting permutation: s1-begin s1-insert s2-begin s2-update-node-1 s1-abort s2-abort
+create_distributed_table
+---------------------------------------------------------------------
+
+(1 row)
+
+step s1-begin: BEGIN;
+step s1-insert: INSERT INTO t1 SELECT generate_series(1, 100);
+step s2-begin: BEGIN;
+step s2-update-node-1:
+    -- update a specific node by address
+    SELECT master_update_node(nodeid, 'localhost', nodeport + 10)
+      FROM pg_dist_node
+     WHERE nodename = 'localhost'
+       AND nodeport = 57637;
+ <waiting ...>
+step s1-abort: ABORT;
+step s2-update-node-1: <... completed>
+master_update_node
+---------------------------------------------------------------------
+
+(1 row)
+
+step s2-abort: ABORT;
+master_remove_node
+---------------------------------------------------------------------
+
+
+(2 rows)
+
+
+starting permutation: s1-begin s1-insert s2-begin s2-update-node-1-force s2-abort s1-abort
+create_distributed_table
+---------------------------------------------------------------------
+
+(1 row)
+
+step s1-begin: BEGIN;
+step s1-insert: INSERT INTO t1 SELECT generate_series(1, 100);
+step s2-begin: BEGIN;
+step s2-update-node-1-force:
+    -- update a specific node by address (force)
+    SELECT master_update_node(nodeid, 'localhost', nodeport + 10, force => true, lock_cooldown => 100)
+      FROM pg_dist_node
+     WHERE nodename = 'localhost'
+       AND nodeport = 57637;
+ <waiting ...>
+step s2-update-node-1-force: <... completed>
+master_update_node
+---------------------------------------------------------------------
+
+(1 row)
+
+step s2-abort: ABORT;
+step s1-abort: ABORT;
+FATAL:  terminating connection due to administrator command
+server closed the connection unexpectedly
+	This probably means the server terminated abnormally
+	before or while processing the request.
+
+master_remove_node
+---------------------------------------------------------------------
+
+
+(2 rows)
+