Run the long list of functional checks that CI runs for V itself.

Usage:
  v test-all [compiler flags...]

The suite compiles and runs examples in a variety of configurations
(C, JS, WASM, cross-compilation, different C compilers, GC modes,
live, etc.), runs a full `v test` over `vlib/builtin`, checks
expression mode with `v -e`, and several other features and
invariants of the compiler.

Compiler options given to `v test-all`, or set through VFLAGS, are
applied to all of the checks.

Passing `-new-compiler` (and not `-old-compiler`) selects strict V3
mode: checks that are not compatible with the V3 compiler are then
skipped, and the remaining ones are run with exactly the given
options.
