fix side effects in allocation tests

Author: IanButterworth

test/misc.jl

@@ -1392,10 +1392,12 @@ end
 @testset "Base/timing.jl" begin
     @test Base.jit_total_bytes() >= 0
 
-    # sanity check `@allocations` returns what we expect in some very simple cases
-    @test (@allocations "a") == 0
-    @test (@allocations "a" * "b") == 0 # constant propagation
-    @test (@allocations "a" * Base.inferencebarrier("b")) == 1
+    # sanity check `@allocations` returns what we expect in some very simple cases.
+    # These are inside functions because `@allocations` uses `Experimental.@force_compile`
+    # so can be affected by other code in the same scope.
+    @test (() -> @allocations "a")() == 0
+    @test (() -> @allocations "a" * "b")() == 0 # constant propagation
+    @test (() -> @allocations "a" * Base.inferencebarrier("b"))() == 1
 
     lock_conflicts() = parse(Int, read(`$(Base.julia_cmd()) -tauto -E '
         @lock_conflicts begin