Confusing parse error about test_proc impl

[mikex-oss]

Describe the bug
The parser/formatter provides an error explaining that impls are not supported in test procs, even though that's not related to the actual issue.

To Reproduce

#[test_proc]
proc foo_test { }

Formatting failed: Error: INVALID_ARGUMENT: ParseError:test.x:1:13-2:18 Test proc with impl is not yet supported at test.x:1:13-2:18
=== Source Location Trace: ===
xls/dslx/frontend/bindings.h:58
xls/dslx/frontend/parser.cc:416
xls/dslx/parse_and_typecheck.cc:71
xls/dslx/dslx_fmt.cc:91

Expected behavior
I expect the real issue to be that config/init/next are not defined. The correct error is provided when specifying at least one of these:

#[test_proc]
proc foo_test {

  config() {}

 }

Formatting failed: Error: INVALID_ARGUMENT: ParseError: test.x:2:1-6:2 Procs must define init, config and next functions; missing: init, next.
=== Source Location Trace: ===
xls/dslx/frontend/bindings.h:58
xls/dslx/frontend/parser.cc:3557
xls/dslx/frontend/parser.cc:416
xls/dslx/parse_and_typecheck.cc:71
xls/dslx/dslx_fmt.cc:91