[SYSTEMDS-3889] Fix new matrix-scalar rewrite test (missing cast)

This patch fixes issues of the test dml scripts in terms of missing
casts from 1-x-1 matrices to scalars. Interestingly, the test ran
fine in local environments because the parser validation runs
differently, and subsequently these 1-x-1 matrices where automatically
rewritten to scalars.

Author: mboehm7

src/test/scripts/functions/rewrite/RewriteScalarMinusMatrixMinusScalar.dml

@@ -19,8 +19,8 @@
 #
 #-------------------------------------------------------------
 A = read($1);
-a = read($2);
-b = read($3);
+a = as.scalar(read($2));
+b = as.scalar(read($3));
 
 R = a - A - b;