build: limit test concurrency

TravisCI and AppVeyor run the tests in very slow VMs.
Some of our tests can't cope with that. Running less tests
in parallel should make them somewhat less flakey.

build/ci.go

@@ -227,6 +227,9 @@ func doTest(cmdline []string) {
 
 	// Run the actual tests.
 	gotest := goTool("test")
+	// Test a single package at a time. CI builders are slow
+	// and some tests run into timeouts under load.
+	gotest.Args = append(gotest.Args, "-p", "1")
 	if *coverage {
 		gotest.Args = append(gotest.Args, "-covermode=atomic", "-cover")
 	}