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

This is a reland of 2c1e4aa9

Original change's description:
> [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/1105050
> Reviewed-by: Jakob Gruber <jgruber@chromium.org>
> Commit-Queue: Michael Achenbach <machenbach@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#53819}

TBR=jgruber@chromium.org

Bug: v8:7853
Change-Id: I21d82ea907c0beead4ad2bc90fecc4dc9036dc1b
Reviewed-on: https://chromium-review.googlesource.com/1109418Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53906}

src/flag-definitions.h

@@ -1142,7 +1142,7 @@ 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