[test] check for availability before testing

- These tests verify updated behaviour of existing API, and should check for availability first.

Author: glessard

test/stdlib/StringAPICString.swift

@@ -226,6 +226,7 @@ CStringTests.test("Substring.withCString") {
 }
 
 CStringTests.test("String.cString.with.Array.UInt8.input") {
+  guard #available(SwiftStdlib 5.7, *) else { return }
   do {
     let (u8p, dealloc) = getASCIIUTF8()
     defer { dealloc() }
@@ -248,6 +249,7 @@ CStringTests.test("String.cString.with.Array.UInt8.input") {
 }
 
 CStringTests.test("String.cString.with.Array.CChar.input") {
+  guard #available(SwiftStdlib 5.7, *) else { return }
   do {
     let (u8p, dealloc) = getASCIIUTF8()
     defer { dealloc() }
@@ -270,6 +272,7 @@ CStringTests.test("String.cString.with.Array.CChar.input") {
 }
 
 CStringTests.test("String.cString.with.String.input") {
+  guard #available(SwiftStdlib 5.7, *) else { return }
   let (u8p, dealloc) = getASCIIUTF8()
   defer { dealloc() }
   var str = String(cString: "ab")
@@ -283,6 +286,7 @@ CStringTests.test("String.cString.with.String.input") {
 }
 
 CStringTests.test("String.cString.with.inout.UInt8.conversion") {
+  guard #available(SwiftStdlib 5.7, *) else { return }
   var c = UInt8.zero
   var str = String(cString: &c)
   expectTrue(str.isEmpty)
@@ -296,6 +300,7 @@ CStringTests.test("String.cString.with.inout.UInt8.conversion") {
 }
 
 CStringTests.test("String.cString.with.inout.CChar.conversion") {
+  guard #available(SwiftStdlib 5.7, *) else { return }
   var c = CChar.zero
   var str = String(cString: &c)
   expectTrue(str.isEmpty)
@@ -309,6 +314,7 @@ CStringTests.test("String.cString.with.inout.CChar.conversion") {
 }
 
 CStringTests.test("String.validatingUTF8.with.Array.input") {
+  guard #available(SwiftStdlib 5.7, *) else { return }
   do {
     let (u8p, dealloc) = getASCIIUTF8()
     defer { dealloc() }
@@ -332,6 +338,7 @@ CStringTests.test("String.validatingUTF8.with.Array.input") {
 }
 
 CStringTests.test("String.validatingUTF8.with.String.input") {
+  guard #available(SwiftStdlib 5.7, *) else { return }
   let (u8p, dealloc) = getASCIIUTF8()
   defer { dealloc() }
   var str = String(validatingUTF8: "ab")
@@ -347,6 +354,7 @@ CStringTests.test("String.validatingUTF8.with.String.input") {
 }
 
 CStringTests.test("String.validatingUTF8.with.inout.conversion") {
+  guard #available(SwiftStdlib 5.7, *) else { return }
   var c = CChar.zero
   var str = String(validatingUTF8: &c)
   expectNotNil(str)
@@ -361,6 +369,7 @@ CStringTests.test("String.validatingUTF8.with.inout.conversion") {
 }
 
 CStringTests.test("String.decodeCString.with.Array.input") {
+  guard #available(SwiftStdlib 5.7, *) else { return }
   do {
     let (u8p, dealloc) = getASCIIUTF8()
     defer { dealloc() }
@@ -385,6 +394,7 @@ CStringTests.test("String.decodeCString.with.Array.input") {
 }
 
 CStringTests.test("String.decodeCString.with.String.input") {
+  guard #available(SwiftStdlib 5.7, *) else { return }
   let (u8p, dealloc) = getASCIIUTF8()
   defer { dealloc() }
   var result = String.decodeCString(
@@ -404,6 +414,7 @@ CStringTests.test("String.decodeCString.with.String.input") {
 }
 
 CStringTests.test("String.decodeCString.with.inout.conversion") {
+  guard #available(SwiftStdlib 5.7, *) else { return }
   var c = Unicode.UTF8.CodeUnit.zero
   var result = String.decodeCString(
     &c, as: Unicode.UTF8.self, repairingInvalidCodeUnits: true
@@ -421,6 +432,7 @@ CStringTests.test("String.decodeCString.with.inout.conversion") {
 }
 
 CStringTests.test("String.init.decodingCString.with.Array.input") {
+  guard #available(SwiftStdlib 5.7, *) else { return }
   do {
     let (u8p, dealloc) = getASCIIUTF8()
     defer { dealloc() }
@@ -443,6 +455,7 @@ CStringTests.test("String.init.decodingCString.with.Array.input") {
 }
 
 CStringTests.test("String.init.decodingCString.with.String.input") {
+  guard #available(SwiftStdlib 5.7, *) else { return }
   let (u8p, dealloc) = getASCIIUTF8()
   defer { dealloc() }
   var str = String(decodingCString: "ab", as: Unicode.UTF8.self)
@@ -456,6 +469,7 @@ CStringTests.test("String.init.decodingCString.with.String.input") {
 }
 
 CStringTests.test("String.init.decodingCString.with.inout.conversion") {
+  guard #available(SwiftStdlib 5.7, *) else { return }
   var c = Unicode.UTF8.CodeUnit.zero
   var str = String(decodingCString: &c, as: Unicode.UTF8.self)
   expectEqual(str.isEmpty, true)