Add a test that methods are not overwritten in test modules

test/runtests.jl

@@ -22,6 +22,8 @@
 # If present, the tests run only those submodules defined above. `General` is
 # not a submodule, but it runs all of the top-level tests in MOI.
 
+import Test
+
 # This file gets called first. If it doesn't crash, all is well.
 include("issue980.jl")
 
@@ -31,8 +33,25 @@ MODULES_TO_TEST = get(
     "General;Benchmarks;Bridges;Bridges/Constraint;Bridges/Objective;Bridges/Variable;FileFormats;Nonlinear;Test;Utilities",
 )
 
+function include_with_method_redefinition_check(jl_filename)
+    stderr_filename = tempname()
+    open(stderr_filename, "w") do io
+        return redirect_stderr(() -> include(jl_filename), io)
+    end
+    contents = read(stderr_filename, String)
+    print(stderr, contents)
+    warnings = findall(
+        r"WARNING: Method definition (.+?) in module (.+?) at (.+?) overwritten at (.+?)\n",
+        contents,
+    )
+    if isempty(warnings)
+        error("Found overwritten method")
+    end
+    return
+end
+
 for submodule in split(MODULES_TO_TEST, ";")
-    include("$(submodule)/runtests.jl")
+    include_with_method_redefinition_check("$submodule/runtests")
     GC.gc()  # Force GC run here to reduce memory pressure
 end