testing: enable examples on js/wasm with non os.Pipe runExample

os.Pipe is not implemented on wasm/js so for that purpose use
a temporary file for js/wasm. This change creates two versions
of runExample:

* runExample verbatim that still uses os.Pipe for non js/wasm
* runExample that uses a temporary file

Also added a TODO to re-unify these function versions back into
example.go wasm/js gets an os.Pipe implementation.

Change-Id: I9f418a49b2c397e1667724c7442b7bbe8942225e
Reviewed-on: https://go-review.googlesource.com/c/go/+/165357


Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>