Cover the remaining dynamic call targets for unhandled errors

[jcmfernandes]

What does this PR do?

Test-only follow-up to the runtime check for unhandled errors from
dynamically-dispatched ! calls.

The spec file that landed with that check covers only two of the dynamic
targets a Starlark program can build, and the Go test covers only a variable
bound to an error-returning builtin. This adds the missing ones:

• spec/optional/error_handling/dynamic_calls.star — a parameter
  (def f(g): g()) and the result of another call (get_handler()()),
  and a header comment that lists them.
• starlark/eval_test.go — an attribute of a module-like value
  (fs.read_all(...), via starlarkstruct), and a bare call at module top
  level, where the pending error is dropped by the module frame's own return
  rather than a function's.
• starlark/testdata/dynamic_calls.star — the attribute target exercised from
  Starlark, where the callee behind the struct field is an ordinary !
  function rather than a builtin, so the pending error reaches the caller's
  frame by the other route. The spec suite cannot express this (its predeclared
  vocabulary is assert, trap, matches, freeze, with no way to build an
  attribute-bearing value); the package testdata environment predeclares
  struct, so it belongs there.

Motivation

fs.read_all("//path/to/missing.yml") — an attribute of an embedder module —
is the shape that motivated the runtime check in the first place, and it had no
test. An attribute target is never statically resolvable, so the resolver
always defers to runtime; it is the case the silent drop hit hardest, and the
one most likely to regress unnoticed.

A parameter and a call result are the other two targets expressible in Starlark
alone. Per spec/harness.md, the attribute case cannot live in spec/, which
is why the coverage is split across the three files above.

Additional notes

No production code changes.

How to test the change?

go test ./...

Verified in both directions:

	at d44dbe2 (check present)	at 1e50f97 (before the check)
go test ./...	pass, 12 packages	—
spec/.../dynamic_calls.star	pass	5 assert.fails cases fail
starlark/testdata/dynamic_calls.star	pass	2 assert.fails cases fail
TestUnhandledDynamicErrorReturningCallFails	4/4 subtests pass	4/4 subtests fail

Before the check, both new Go cases return None and no error at all:
globals = {result: None, spec: None}.

PR stack

• master
  • Cover the remaining dynamic call targets for unhandled errors 👈