strings: Add benchmark test for trim function

The strings.Trim function and variants allocate memory on the heap when creating a function to pass into TrimFunc.
Add a benchmark to document the behavior; an issue will be submitted to address this behavior in the compiler if possible.

Change-Id: I8b66721f077951f7e7b8cf3cf346fac27a9b68c0
Reviewed-on: https://go-review.googlesource.com/8200Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/strings/strings_test.go

@@ -569,6 +569,35 @@ func TestTrim(t *testing.T) {
 	}
 }
 
+func BenchmarkTrim(b *testing.B) {
+	b.ReportAllocs()
+
+	for i := 0; i < b.N; i++ {
+		for _, tc := range trimTests {
+			name := tc.f
+			var f func(string, string) string
+			switch name {
+			case "Trim":
+				f = Trim
+			case "TrimLeft":
+				f = TrimLeft
+			case "TrimRight":
+				f = TrimRight
+			case "TrimPrefix":
+				f = TrimPrefix
+			case "TrimSuffix":
+				f = TrimSuffix
+			default:
+				b.Errorf("Undefined trim function %s", name)
+			}
+			actual := f(tc.in, tc.arg)
+			if actual != tc.out {
+				b.Errorf("%s(%q, %q) = %q; want %q", name, tc.in, tc.arg, actual, tc.out)
+			}
+		}
+	}
+}
+
 type predicate struct {
 	f    func(rune) bool
 	name string