Run the full test suite for the V compiler and standard library.

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

The command runs the full set of `*_test.v` files for the V compiler
modules, the standard library and the runtime, in a single test
session, reporting a summary at the end.

Compiler options given to `v test-self`, or set through VFLAGS, are
passed to every test compilation. The collection of the compiler tests
in `vlib/v/` has its own dedicated test driver, so those tests are
excluded here.

Environment variables used by CI:
  VTEST_SELF_SHARD_COUNT / VTEST_SELF_SHARD_INDEX
        split the suite deterministically into several shards, so that
        multiple machines can test it in parallel; the assignment is
        based on a hash of each test file's path.
  VTEST_JUST_ESSENTIAL
        reduce the suite to its essential test files only.
