test(cl): when expect.txt is ';', skip diff only, not execution

[luoliwoshang]

Background

In cl runtime tests, many cases use a single ; in expect.txt. In practice this is often used for outputs that are not stable for textual golden comparison (for example, address-like values or environment-dependent formatting).

However, the current implementation treats this marker as skip the whole run, so tests appear green while the case is never executed.

Current Behavior

In cl/cltest/cltest.go (testRunFrom):

if bytes.Equal(expected, []byte{';'}) { // expected == ";" means skipping expect.txt
    return
}

This returns before RunAndCaptureWithConf(...).

Why This Is a Problem

expect.txt == ";" effectively means “do not run”, not just “do not diff output”.

Consequences:

• build/runtime regressions can be hidden in go test ./cl;
• false confidence: CI reports pass while manual llgo run fails for the same case.

Expected Behavior

For expect.txt == ";":

• still execute RunAndCaptureWithConf(...) (at least validate build/run success);
• skip only output vs expect.txt textual diff.

If we still need “skip execution entirely”, introduce a separate explicit marker.

Reproduction Example

cd cl/_testgo/cgobasic
llgo run -target esp32c3-basic -emulator .

This fails with build/SSA errors, while the current go test ./cl path may not execute this case when expect.txt == ";".

---

If helpful, I can follow up with a PR that changes ; to run-only/no-diff semantics and adds a regression test.