fs/asyncreader: skip some tests to work around race detector bug

The race detector currently detects a race with len(chan) against
close(chan).

See: https://github.com/golang/go/issues/27070

Skip the tests which trip this bug under the race detector.

lib/israce/israce.go

@@ -0,0 +1,9 @@
+// +build race
+
+// Package israce reports if the Go race detector is enabled.
+//
+// From https://stackoverflow.com/questions/44944959/how-can-i-check-if-the-race-detector-is-enabled-at-runtime
+package israce
+
+// Enabled reports if the race detector is enabled.
+const Enabled = true

lib/israce/norace.go

@@ -0,0 +1,9 @@
+// +build !race
+
+// Package israce reports if the Go race detector is enabled.
+//
+// From https://stackoverflow.com/questions/44944959/how-can-i-check-if-the-race-detector-is-enabled-at-runtime
+package israce
+
+// Enabled reports if the race detector is enabled.
+const Enabled = false