Improve handling of -short flag in tests (#9075)

As a small developer quality of life improvement, I found many individual unit tests that take longer than around a second to complete, and set them to skip when run under `go test -short`.

On my machine, the wall timings for tests (with `go test -count=1 ./...` and optionally `-short` and `-race`) are roughly:

- Long tests, no race detector: about 1m42s
- Short tests, no race detector: about 17s
- Long tests, race detector enabled: about 2m1s
- Short tests, race detector enabled: about 28s

This PR is split into many commits each touching a single package, with commit messages detailing the approximate timing change per package.

internal/mempool/mempool_test.go

@@ -132,6 +132,10 @@ func convertTex(in []testTx) types.Txs {
 }
 
 func TestTxMempool_TxsAvailable(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping test in short mode")
+	}
+
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 
@@ -540,6 +544,10 @@ func TestTxMempool_CheckTxSameSender(t *testing.T) {
 }
 
 func TestTxMempool_ConcurrentTxs(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping test in short mode")
+	}
+
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()