Fix a few testing errors with promotedSizeMismatch (#26622)

[trivial, not reviewed]

First, I made the classic mistake of failing to think about --fast
testing, which turns off the bounds checking this test is looking for.
Here, I add --checks to its .compopts file to try and avoid this going
forward.

More subtly, it didn't occur to me that the test has a race since the
leader will fire off multiple follower tasks, giving the potential to
have the size mismatch in dimension 0 or 1 to appear first. Here, I
side-stepped that by only making a single dimension mismatch in size.

test/arrays/errors/promotedSizeMismatch.chpl

@@ -1,5 +1,5 @@
 var A : [1..5, 1..5] int = 0;
-var B : [1..3, 1..3] int = 1;
+var B : [1..5, 1..3] int = 1;
 
 A += B;
 

test/arrays/errors/promotedSizeMismatch.compopts

@@ -0,0 +1 @@
+--checks