context: clean up test files

Tests in package context cannot depend directly on package testing due to an import cycle.
We resolved this by having test functions in package context_test (x_test.go) forward to
test functions in package context (context_test.go). This is fragile, since it's easy
to add a test to context_test.go and forget to add the forwarding function, and tests
written in this way cannot easily use testing package features like t.Run for subtests.

It turns out that only four test functions actually use unexported members of package
context. This CL moves all except those four to x_test.go and makes them regular tests.
It also updates TestCause to use t.Run and t.Parallel to parallelize its test cases.
It also adds documentation indicating when tests should be added to each file.

Change-Id: Ic60bae32a7a44e07831b5388c9af219d53ba9af3
Reviewed-on: https://go-review.googlesource.com/c/go/+/480375
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Sameer Ajmani <sameer@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>

1 files changed, 0 insertions, 2 deletions

diff --git a/src/context/example_test.go b/src/context/example_test.go
index 7779f5f1b3..8dad0a4220 100644
--- a/src/context/example_test.go
+++ b/src/context/example_test.go
@@ -10,8 +10,6 @@ import (
 	"time"
 )
 
-const shortDuration = 1 * time.Millisecond // a reasonable duration to block in an example
-
 var neverReady = make(chan struct{}) // never closed
 
 // This example demonstrates the use of a cancelable context to prevent a