cmd/cgo/internal/test: fix TestThreadLock
This test was introduced in CL 18882, but only recently enabled as of
CL 493603. It's intended to check that we don't move executing C code
between threads when it re-enters Go, but it has always contained a
flake. Go *can* preempt between the Go call to gettid and the C call
to gettid and move the goroutine to another thread because there's no
C code on the stack during the Go call to gettid. This will cause the
test to fail.
Fix this by making both gettid calls in C, with a re-entry to Go
between them.
Fixes #60265
Change-Id: I546621a541ce52b996d68b17d3bed709d2b5b1f3
Reviewed-on: https://go-review.googlesource.com/c/go/+/496182
Auto-Submit: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Austin Clements <austin@google.com>