test: add tests for string/[]byte/[]rune conversions

Matches examples in spec section on string conversions.

For #23814.

Change-Id: I08099c27bfdb98735868266f5a42901321b97b56
Reviewed-on: https://go-review.googlesource.com/c/go/+/412095
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>

test/fixedbugs/issue23814.go

@@ -0,0 +1,61 @@
+// run
+
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Examples from the language spec section on string conversions.
+
+package main
+
+func main() {
+	// 1
+	_ = string('a')  // "a"
+	_ = string(-1)   // "\ufffd" == "\xef\xbf\xbd"
+	_ = string(0xf8) // "\u00f8" == "ø" == "\xc3\xb8"
+
+	type myString string
+	_ = myString(0x65e5) // "\u65e5" == "日" == "\xe6\x97\xa5"
+
+	// 2
+	_ = string([]byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
+	_ = string([]byte{})                                   // ""
+	_ = string([]byte(nil))                                // ""
+
+	type bytes []byte
+	_ = string(bytes{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
+
+	type myByte byte
+	_ = string([]myByte{'w', 'o', 'r', 'l', 'd', '!'})     // "world!"
+	_ = myString([]myByte{'\xf0', '\x9f', '\x8c', '\x8d'}) // "🌍
+
+	// 3
+	_ = string([]rune{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
+	_ = string([]rune{})                       // ""
+	_ = string([]rune(nil))                    // ""
+
+	type runes []rune
+	_ = string(runes{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
+
+	type myRune rune
+	_ = string([]myRune{0x266b, 0x266c}) // "\u266b\u266c" == "♫♬"
+	_ = myString([]myRune{0x1f30e})      // "\U0001f30e" == "🌎
+
+	// 4
+	_ = []byte("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
+	_ = []byte("")      // []byte{}
+
+	_ = bytes("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
+
+	_ = []myByte("world!")      // []myByte{'w', 'o', 'r', 'l', 'd', '!'}
+	_ = []myByte(myString("🌏")) // []myByte{'\xf0', '\x9f', '\x8c', '\x8f'}
+
+	// 5
+	_ = []rune(myString("白鵬翔")) // []rune{0x767d, 0x9d6c, 0x7fd4}
+	_ = []rune("")              // []rune{}
+
+	_ = runes("白鵬翔") // []rune{0x767d, 0x9d6c, 0x7fd4}
+
+	_ = []myRune("♫♬")          // []myRune{0x266b, 0x266c}
+	_ = []myRune(myString("🌐")) // []myRune{0x1f310}
+}