make `raco test' more like `racket'

Instad of `(dynamic-require .. #f)', use `(dynamic-require .. 0)', which
has the effect of making compile-time code "available" (see docs) in
case the loaded module uses `eval' on syntax objects that refer to
non-kernel syntax.
add v5.3.3 notes
restore changes that don't go into 5.3.3
2d
Increase timeout for TR random testing.
preserve lexical context, src, etc for sequence syntax expansion

The previous version of the code replaced the implicit #%app with
the kernel #%app (ie, #%plain-app), causing sequence functions
that take keyword arguments to fail.
Post-release version for the v5.3.3 release
dont use raco test for encode.rkt (as it expects command-line
arguments and doesn't have a test submodule)
change raco test's announcements to say "raco test"
move dir-chars.rkt to unstable/2d to unbreak dependencies
tweak raco test output, yet again
unbreak plai random mutator tests