test(eval): avoid ETXTBSY in parallel fake-runtime tests

[dgageot]

Summary

Fix the existing Linux flake exposed by #4554: TestRunDockerAgentInContainerUsesConfiguredRuntime failed to execute its freshly written fake-podman script with text file busy (CI log).

The affected tests write executable scripts after entering t.Parallel(). A concurrent fork can inherit a writable descriptor until its child reaches exec, causing Linux to reject execution with ETXTBSY even after os.WriteFile returns.

• Create fake-runtime executables before the top-level parallel barrier, following the existing build_test.go pattern.
• Use shared immutable scripts and per-case symlinks in table-driven tests; script output remains isolated in each subtest's directory.
• Fix the same pattern across the other affected evaluation tests. No production code changes.

Related to #4554; this does not modify or close that dependency-upgrade PR.

Validation

• task build
• task test
• task lint using official golangci-lint v2.13.2 built with Go 1.27.0
• Entire pkg/evaluation test binary: 100 repetitions with -test.shuffle=on -test.parallel=32 on Linux/arm64 in Docker, built with Go 1.27.0

The baseline stress run did not reproduce the flake. Local Linux validation was arm64, not CI's amd64; CI validation is pending.