[tests] Skip slow test debug-step-prefix-bytecodes in debug mode

It's been regularly timing out on the Linux debug bot.

TBR=machenbach@chromium.org

Change-Id: I250fc55acb5d714c7060edb691eb9759a7d38a15
Reviewed-on: https://chromium-review.googlesource.com/568794Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46606}

test/debugger/debugger.status

@@ -24,7 +24,7 @@
   # Slow tests
   'debug/debug-scopes': [PASS, SLOW],
   'debug/debug-stepout-scope-part*': [PASS, SLOW],
-  'debug/ignition/debug-step-prefix-bytecodes': [PASS, SLOW],
+  'debug/ignition/debug-step-prefix-bytecodes': [PASS, SLOW, ['mode == debug', SKIP]],
 
   # Too slow in debug mode and on slow platforms.
   'regress/regress-2318': [PASS, SLOW, ['mode == debug or (arch != ia32 and arch != x64) or asan == True or msan == True', SKIP]],