build: derive TEST_BINS from the build rules, not the .c files (corrects #731)

[JustVugg]

Correction to #731, found by merging dev into a real contributor branch rather than by reasoning about it.

What #731 got wrong

#731 replaced the hand-written TEST_BINS line with a glob of tests/test_*.c. That was wrong in the opposite direction from the list it replaced: it promotes any test_*.c file into a gate, including ones that deliberately have no build rule.

On dev the four such files were known and excluded by name, so the resulting set was identical and the mistake was invisible. On a contributor branch it is not. Merging dev into #529 promoted that branch's tests/test_fp8_e2e_loader.c — a file @monotophic has but never gated — and the build failed:

test_fp8_e2e_loader.c:(.text+0x4c5cd): undefined reference to `sqrtf'
collect2: error: ld returned 1 exit status

Every future branch carrying an un-wired test would have hit the same thing at merge time.

The fix

Having a build rule is the honest definition of a gate, so derive the list from the rules instead of from the files. TEST_EXCLUDE drops from five entries to one — test_uring, which has a rule but is Linux-only and is appended conditionally just below.

The property that motivated all of this is unchanged and now stronger: adding a gate means adding your .c and its own rule, which land in different places in the file. There is no shared list left to conflict on at all.

Verification

• Identical to current dev: 31 entries, same names.
• make test-c green.
• Re-ran the formats: self-describing container stamp (TRUST-VERIFY-REFUSE) + FORMATS.md registry (#524) #529 merge with this in place: 34 gates — @monotophic's test_fp8_load, test_fp8_passthrough and test_qt_addrow all included (they have rules), and test_fp8_e2e_loader correctly left out.

One file, build system only.