proposal: testing: add Test function

[rogpeppe]

The testing package has long contained the Benchmark function, which allows code in a non-testing source file to run benchmarks.

It is sometimes useful to be able to run test code in a non-testing context too; for example to demonstrate a failing test in the Go playground.

Perhaps something like this?

// Test runs f as a test with the given name. Any test output
// is written to w. Test reports whether the test has succeeded.
func Test(w io.Writer, name string, f func(t *testing.T)) bool

[josharian]

See also #6511

There is work in progress now to allow multiple files in the playground, which should make this better yet.