go/analysis/passes/modernize: testingcontext - skip deferred calls

In the testingcontext modernizer, we replace calls to context.WithCancel
with a use of testing.{T,B,F}.Context(). However, if there are deferred
calls before the call to context.WithCancel(), this replacement will
result in a different order of execution for the deferred calls:

func TestFunc(t *testing.T) {
 ...
 defer funcA()
 ctx, cancel := context.WithCancel(...)
 defer cancel()
 ...
}

=>

func TestFunc(t *testing.T) {
 ...
 defer funcA()
 ctx := t.Context()
 ...
}

In the first case, the deferred cancel() executes first,
then funcA().

In the second case, cancel() is executed after TestFunc and
all its deferred functions complete. So deferA() executes
before cancel(). This change may have side effects
that affect program behavior.

To avoid altering the order that deferred functions are
executed, we should not suggest a fix when there is a
deferred call before the call to context.WithCancel.

Also, change the TextEdits to preserve comments
between the assignment and the defer.

Fixes golang/go#80989

Change-Id: I5f83f981c45e8990c5db7f0c4d2ad8a036fa4b5c
Reviewed-on: https://go-review.googlesource.com/c/tools/+/821202
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Madeline Kalil <mkalil@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alex Putman <aputman@golang.org>