x/exp/typeparams: use regexp to match wanted result

This is a port of CL 428055 to the external x/exp/typeparams package.
Also fix a stale test error message.

Fixes golang/go#54903

Change-Id: Iafa0df489e1d2bf7ce43f2e7854d87b2635e1f42
Reviewed-on: https://go-review.googlesource.com/c/exp/+/428875
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>

Author: findleyr

typeparams/normalize_test.go

@@ -9,6 +9,7 @@ import (
 	"go/parser"
 	"go/token"
 	"go/types"
+	"regexp"
 	"strings"
 	"testing"
 
@@ -36,7 +37,7 @@ func TestNormalTerms(t *testing.T) {
 		{"package emptyintersection; type T[P interface{ ~int; string }] int", "", "empty type set"},
 
 		{"package embedded0; type T[P interface{ I }] int; type I interface { int }", "int", ""},
-		{"package embedded1; type T[P interface{ I | string }] int; type I interface{ int | ~string }", "int|~string", ""},
+		{"package embedded1; type T[P interface{ I | string }] int; type I interface{ int | ~string }", "int ?\\| ?~string", ""},
 		{"package embedded2; type T[P interface{ I; string }] int; type I interface{ int | ~string }", "string", ""},
 
 		{"package named; type T[P C] int; type C interface{ ~int|int }", "~int", ""},
@@ -50,7 +51,7 @@ type B interface{ int|string }
 type C interface { ~string|~int }
 
 type T[P interface{ A|B; C }] int
-`, "~string|int", ""},
+`, "~string ?\\| ?int", ""},
 	}
 
 	for _, test := range tests {
@@ -94,8 +95,9 @@ type T[P interface{ A|B; C }] int
 				qf := types.RelativeTo(pkg)
 				got = types.TypeString(NewUnion(terms), qf)
 			}
-			if got != test.want {
-				t.Errorf("StructuralTerms(%s) = %q, want %q", T, got, test.want)
+			want := regexp.MustCompile(test.want)
+			if !want.MatchString(got) {
+				t.Errorf("NormalTerms(%s) = %q, want matching %q", T, got, test.want)
 			}
 		})
 	}