cmd/api: move support checks into individual tests

cmd/api: move support checks into individual tests

This makes 'go test -list cmd/api' work, and fixes an infinite
recursion via testenv.HasExec that would otherwise occur.
As of CL 488076, testenv.HasExec tries to re-exec the test
executable using -list to suppress running the tests, which
produces a fork bomb if TestMain itself calls HasExec.

For this test, it turns out that the HasExec check is redundant
anyway: if we can exec 'go build', we can certainly exec programs in
general too.

Change-Id: I165f98315c181098c8be8b7525b9dfa3f98e14f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/491656
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>