[test] Initialize slow-dcheck runtime flag with compile time value

This makes sure slow dchecks are part of the snapshot when the compile
time flag is set. It'll also obviate the need to set the runtime flag
in general, making configurations simpler.

It's still possible to opt out at runtime for the non-snapshot dchecks
using the negated runtime flag.

Bug: v8:7853
Change-Id: Iadfc46ed691d32e9d39717e2ea0383e784660d1d
Reviewed-on: https://chromium-review.googlesource.com/1105050Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53819}

src/flag-definitions.h

@@ -1136,7 +1136,7 @@ DEFINE_NEG_IMPLICATION(gdbjit, compact_code_space)
 
 // checks.cc
 #ifdef ENABLE_SLOW_DCHECKS
-DEFINE_BOOL(enable_slow_asserts, false,
+DEFINE_BOOL(enable_slow_asserts, true,
             "enable asserts that are slow to execute")
 #endif