cmd/testwrapper, tstest: move test sharding out of test code

Previously, sharding required tests to opt in by calling tstest.Shard,
which used a process-global counter to assign each test to a shard.
This had two problems: most tests didn't call it, so they ran on every
shard (defeating the purpose), and shard assignments were unstable
(depended on call order, so adding a test could reshuffle others).

Remove tstest.Shard and tstest.SkipOnUnshardedCI entirely. Instead,
have testwrapper implement sharding automatically for all tests: when
TS_TEST_SHARD=N/M is set, it uses "go list -json" (no compilation) to
find test source files, scans them for top-level Test/Benchmark/
Example/Fuzz function names, and filters by fnv32a(name) % M == N-1.
The filtered names are passed as an anchored -run regex to go test.

Using go list instead of "go test -list" avoids linking the test binary
twice (Go's build cache does not cache test binary linking).

Fixes #19886

Change-Id: I62ab7b3d757324d4c5fd0b5de50c1e3742681791
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/largetailnet/delta_test.go

@@ -57,7 +57,6 @@ func metricByName(t testing.TB, name string) *clientmetric.Metric {
 // [tstest/largetailnet/BenchmarkGiantTailnet], which only measures cost
 // of the same fast path — this test verifies correctness.
 func TestNetmapDeltaFastPath(t *testing.T) {
-	tstest.Shard(t)
 
 	logf := logger.Discard
 	if testing.Verbose() {
@@ -105,7 +104,7 @@ func TestNetmapDeltaFastPath(t *testing.T) {
 	// Snapshot baseline metric values; we'll assert deltas against
 	// these. Globals make per-test isolation impossible, but deltas
 	// are robust against interleaving (assuming no other test runs in
-	// parallel here, hence tstest.Shard above).
+	// parallel here).
 	mFast := metricByName(t, "controlclient_map_response_handled_incrementally")
 	mFull := metricByName(t, "controlclient_map_response_handled_full_rebuild")
 	mUpsert := metricByName(t, "localbackend_netmap_delta_peer_upserted")