[flang][test] fix sporadically failing test (#134608)

The test is checking output from MLIR debug prints. MLIR passes can be
executed in parallel, for example a pass on func.func might schedule
different func.func operations in different threads. This led to
intermittent test failures where debug output from different threads
became mixed up.

Fix by disabling mlir multithreading for this test.

Author: tblah

flang/test/Transforms/DoConcurrent/loop_nest_test.f90

@@ -3,7 +3,7 @@
 ! REQUIRES: asserts
 
 ! RUN: %flang_fc1 -emit-hlfir  -fopenmp -fdo-concurrent-to-openmp=host \
-! RUN:   -mmlir -debug %s -o - 2> %t.log || true
+! RUN:   -mmlir -debug -mmlir -mlir-disable-threading %s -o - 2> %t.log || true
 
 ! RUN: FileCheck %s < %t.log