Unit test to cover previously-broken read()

This bug managed to survive for 18 years before anyone caught it, so we
should probably try to cover any regressions.  I've verified that this
test now fails if run without the suffix-finding fixes.

Author: roystgnr

tests/numerics/sparse_matrix_test.h

@@ -196,9 +196,12 @@ class SparseMatrixTest : public CppUnit::TestCase
 
     // If we're working with serial matrices then just print one of
     // them so they don't step on the others' toes.
+    //
+    // Use a very short filename, because we had a bug with that and
+    // we want to test it.
     if (matrix->n_processors() > 1 ||
         TestCommWorld->rank() == 0)
-      matrix->print_matlab(libmesh_suite_name+"_matrix.m");
+      matrix->print_matlab("M.m");
 
     matrix->clear();
 
@@ -213,7 +216,7 @@ class SparseMatrixTest : public CppUnit::TestCase
     return;
 #endif
 
-    matrix->read(libmesh_suite_name+"_matrix.m");
+    matrix->read("M.m");
 
     TestCommWorld->barrier();