test: instantiate the reference specializations in the hardened build

[steve-downey]

Follow-up to an incidental finding from #80.

expected_hardened.test.cpp only ever used expected<T, E> and expected<void, E> with a value E. Consequently the precondition checks in the expected<T&, E> specialization were never compiled at all, and neither the primary nor the void specialization was ever instantiated with a reference E. Of the 21 BEMAN_EXPECTED_TRAP() sites in the header, only 15 were reachable by any translation unit in the hardened build — the six in expected<T&, E> were dead as far as the test suite was concerned.

Adds happy-path coverage for expected<T&, E>, expected<T, E&>, expected<T&, E&> and expected<void, E&>, in the same style as the existing sections.

What this does and doesn't buy

The checks are the same if (!has_val_) TRAP shape throughout, so this is mostly about compiling them at all — it is fairly boring, and arguably shouldn't be load-bearing. What it does verify beyond mere instantiation is that the guarded bodies still return the right thing once T or E is a reference: the shallow-const observers in particular, where a const expected still hands out a mutable T* / T& / E&, and where const E& with E = int& collapses back to int&. Those are asserted with static_assert alongside the runtime checks.

Happy paths only, matching the rest of the file: a violated precondition traps, and that cannot be observed from inside the process — __builtin_trap() kills it with SIGILL before anything can report.

Verification

Trap sites present in the gcov data for the hardened TU, before and after:

before: 15  [943 952 961 970 979 988 1051 1060 1069 1078 1977 2000 2009 2018 2027]
after:  21  [... 2973 2982 3022 3031 3040 3049]

Hardened target goes from 5 test cases to 13; full suite 1089/1089 green; clang-format clean.

Independent of #80 — no file overlap. Worth noting for whoever merges second: #80 excludes BEMAN_EXPECTED_TRAP() lines from the gcovr report by pattern, and that pattern already matches all 21 sites, so the two compose without adjustment.

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