testsuite: Enlarge acceptance window for T1969

As noted in #17624, it's quite unstable, especially, for some reason, on
i386 and armv7 (something about 32-bit platforms perhaps?).

Metric Increase:
    T1969

testsuite/tests/perf/compiler/all.T

@@ -9,7 +9,7 @@ setTestOpts(no_lint)
 
 test('T1969',
      [# expect_broken(12437),
-      collect_compiler_residency(15),
+      collect_compiler_residency(20),
       collect_compiler_stats('bytes allocated', 1),
       only_ways(['normal']),