Fix SEGV test

MVM_JIT_LOG no longer complains about failed opens. Cover
for its SEGV only.

t/05-messages/moar/01-errors.t

@@ -7,15 +7,20 @@ plan 1;
 
 # https://github.com/rakudo/rakudo/issues/1451
 subtest 'no SEGV with failed opens of MVM debug files' => {
-    plan +my @vars := <
-        MVM_SPESH_LOG  MVM_JIT_LOG  MVM_DYNVAR_LOG  MVM_COVERAGE_LOG
+    plan 1 + my @vars := <
+        MVM_SPESH_LOG  MVM_DYNVAR_LOG  MVM_COVERAGE_LOG
     >;
-    my $file := (make-temp-dir).absolute;
+    my $dir := (make-temp-dir).absolute;
     for @vars {
-        (temp %*ENV){$_} = $file;
+        (temp %*ENV){$_} = $dir;
         is-run ｢42.say｣, :out(*), :err{.contains: 'Failed to open file'},
             :exitcode(1), $_;
     }
+
+    {
+        (temp %*ENV)<MVM_JIT_LOG> = $dir;
+        is-run ｢42.say｣, :out(*), :err(''), :0exitcode, 'MVM_JIT_LOG';
+    }
 }
 
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