e2e framework: include additional stack backtrace in failures

When a Gomega failure is converted to an error, the stack at the time when the
failure occurs may be useful: error wrapping provides some bread crumbs that
can be followed to determine where the failure really occurred, but error
wrapping may be missing or ambiguous.

To provide the additional information, a FailureError now includes a full stack
backtrace. The backtrace intentionally makes no attempt to exclude framework
functions besides the gomega support itself because helpers like
e2e/framework/pod may be relevant.

That backtrace is not included in the failure message for the sake of
brevity. Instead, it gets logged as part of the test's output.

test/e2e/framework/expect_test.go

@@ -25,6 +25,20 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+// This test is sensitive to line numbering.
+// The following lines can be removed to compensate for import changes.
+//
+//
+//
+//
+//
+//
+//
+//
+//
+//
+// This must be line #40.
+
 func TestNewGomega(t *testing.T) {
 	if err := Gomega().Expect("hello").To(gomega.Equal("hello")); err != nil {
 		t.Errorf("unexpected failure: %s", err.Error())
@@ -38,4 +52,11 @@ not to equal
 	if !errors.Is(err, ErrFailure) {
 		t.Errorf("expected error that is ErrFailure, got %T: %+v", err, err)
 	}
+	var failure FailureError
+	if !errors.As(err, &failure) {
+		t.Errorf("expected error that can be copied to FailureError, got %T: %+v", err, err)
+	} else {
+		assert.Regexp(t, `^k8s.io/kubernetes/test/e2e/framework.TestNewGomega\(0x[0-9A-Fa-f]*\)
+	.*/test/e2e/framework/expect_test.go:46`, failure.Backtrace())
+	}
 }