cmd/dist: add maymorestack tests
    
    These tests run the runtime, reflect, and sync package tests with the
    two maymorestack hooks we have.
    
    These tests only run on the longtest builders (or with
    GO_TEST_SHORT=false) because we're running the runtime test two
    additional times and the mayMoreStackMove hook makes it about twice as
    slow (~230 seconds).
    
    To run just these tests by hand, do
    
      GO_TEST_SHORT=false go tool dist test -run mayMoreStack
    
    Updates #48297.
    
    This detected #49354, which was found as a flake on the dashboard, but
    was reliably reproducible with these tests; and #49395.
    
    Change-Id: If785a8b8d6e1b9ad4d2ae67493b54055ab6cbc85
    Reviewed-on: https://go-review.googlesource.com/c/go/+/361212
    Run-TryBot: Austin Clements <austin@google.com>
    TryBot-Result: Gopher Robot <gobot@golang.org>
    Auto-Submit: Austin Clements <austin@google.com>
    Reviewed-by: Cherry Mui <cherryyz@google.com>