cmd/go: extract the TestScript engine into a standalone package
This change decouples the script engine from both cmd/go and the
testing package; I intend to reuse it in the replacement for the
vcs-test.golang.org server.
This change also adds a few new script commands:
- 'echo' echoes its arguments, useful for verifying argument expansion.
- 'cat' prints the contents of files, useful for debugging failing script tests.
- 'help' displays information about script commands and conditions,
reducing the toil of maintaining lists in the README file.
The 'cmp' and 'cmpenv' commands now use internal/diff instead of their
own separate diff implementation.
The 'env' command now writes to the script log instead of the stdout
buffer. (This makes it more consistent with the behavior of other
synchronous builtins.)
The 'stale' command no longer logs output when a target is
unexpectedly non-stale. (However, the ouput of the 'stale' command is
not usually very useful anyway.)
The 'grep', 'stdout', and 'stderr' commands now display matching lines
(like Unix 'grep'), making their negation behavior more consistent
with running real commands on a command-line.
Likewise, the 'cmp' command now always displays differences. That
makes it useful with the '?' prefix to produce diffs for informational
purposes while debugging.