Speed up unit tests in CI (#5360)

* ci: drop gen-5 instance families from the test runner

m5a runners run the unit suite ~2.3x slower than m8id (15m vs 6.5m for `go test`), and when spot capacity for newer families dries up runs-on falls back to them. Keeping gen-6 through gen-8 leaves a wide spot pool without the slow tail.

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

* test: shrink decompression bomb fixtures

The bomb and heap-bound tests pushed 100MB-16GB through gzip and parsers, which under `-race` made them the slowest tests in the unit suite. Each now uses the smallest fixture that still separates the bounded path from the unbounded one.

- arch mtree caps are measured at a small cap and scaled to the shipped constants, so raising a cap still fails the budget
- snap and debian bombs expand to 32MB instead of 512MB (4x their 8MB budgets)
- golang ELF bombs still declare 256MB but deliver 64MB

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

---------

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

.github/workflows/validations.yaml

@@ -27,7 +27,7 @@ jobs:
     #   enough; a 4 vCPU instance can't pull much more than that anyway.
     #   note: s3-cache intentionally omitted -- PR runs are untrusted and must not write to the
     #   shared cache backend that the trusted release workflow reads from (cache poisoning).
-    runs-on: &test-runner "runs-on=${{ github.run_id }}/cpu=4+8/ram=16+32/family=m5+m6+m7+m8+c5+c6+c7+c8/spot=price-capacity-optimized/volume=100gb"
+    runs-on: &test-runner "runs-on=${{ github.run_id }}/cpu=4+8/ram=16+32/family=m6+m7+m8+c6+c7+c8/spot=price-capacity-optimized/volume=100gb"
     permissions:
       contents: read
     steps: