Run at most 4 tests of a package at once in the test phase (closes #224)

On a busy host, TestService_Get_ReturnsByItsDeadline failed because its
request returned over a second after its 200 ms deadline. The request
does not ignore its deadline: with as many tests running at once in the
package's test process as the host has CPUs, it waited to be scheduled,
or for SQLite calls queued on a lock inside the SQLite library that
every database in the process shares. The test phase now passes
-parallel 4 to go test. -p is unchanged: capping it also caps
compiling, which made the step much slower.

Model: opus-5-5

TODO.md

@@ -30,6 +30,14 @@ P2: security: per-IP rate limiting on the image routes
 
 # Completed Steps
 
+- 2026-10-08 the test phase of the `Dockerfile` runs at most 4 tests of a
+  package at once (`go test -parallel 4`, closes #224), where it ran as many as
+  the host has CPUs. On a busy host, `TestService_Get_ReturnsByItsDeadline`
+  failed because its request returned over a second after its deadline: the
+  request was waiting to be scheduled, or for SQLite calls queued on a lock
+  inside the SQLite library that every database in the test process shares.
+  `-p`, how many packages are tested at once, is unchanged, as capping it also
+  slows compiling.
 - 2026-10-08 every output format is saved with settings pixa sets on purpose
   (closes #232): each format has its own govips export, as JPEG XL does, in
   place of govips' generic `Export`, which sent libvips a zero for some settings