Avoid test failures on hosts that only support IPv4 (ruby#12213)

To verify the behavior of HEv2, some tests were prepared. But unexpected failures occur in certain environments.
This happens in environments where "localhost" resolves only to an IPv4 address during tests that verify connections to IPv6.

For example, the following situation can occur:

- The server process is bound to ::1.
- The client socket always resolves "localhost" to 127.0.0.1 and attempts to connect to 127.0.0.1.
- Since no server is bound to 127.0.0.1, an ECONNREFUSED error is raised.

In such situations, the behavior of `TCPSocket.new` remains unchanged from before the introduction of HEv2.
(The failures occur because tests explicitly binding to ::1 were added to verify HEv2 behavior.)

This change ensures that the affected tests are skipped in environments of this kind.

test/socket/test_tcp.rb

@@ -146,9 +146,11 @@ def test_initialize_v6_hostname_resolved_earlier
     return if RUBY_PLATFORM =~ /mswin|mingw|cygwin/
 
     begin
+      # Verify that "localhost" can be resolved to an IPv6 address
+      Socket.getaddrinfo("localhost", 0, Socket::AF_INET6)
       server = TCPServer.new("::1", 0)
-    rescue Errno::EADDRNOTAVAIL # IPv6 is not supported
-      exit
+    rescue Socket::ResolutionError, Errno::EADDRNOTAVAIL # IPv6 is not supported
+      return
     end
 
     server_thread = Thread.new { server.accept }
@@ -192,9 +194,11 @@ def test_initialize_v6_hostname_resolved_in_resolution_delay
     return if RUBY_PLATFORM =~ /mswin|mingw|cygwin/
 
     begin
+      # Verify that "localhost" can be resolved to an IPv6 address
+      Socket.getaddrinfo("localhost", 0, Socket::AF_INET6)
       server = TCPServer.new("::1", 0)
-    rescue Errno::EADDRNOTAVAIL # IPv6 is not supported
-      exit
+    rescue Socket::ResolutionError, Errno::EADDRNOTAVAIL # IPv6 is not supported
+      return
     end
 
     port = server.addr[1]
@@ -290,7 +294,7 @@ def test_initialize_resolv_timeout_with_connection_failure
     begin
       server = TCPServer.new("::1", 0)
     rescue Errno::EADDRNOTAVAIL # IPv6 is not supported
-      exit
+      return
     end
 
     port = server.connect_address.ip_port
@@ -314,8 +318,9 @@ def test_initialize_with_hostname_resolution_failure_after_connection_failure
     begin
       server = TCPServer.new("::1", 0)
     rescue Errno::EADDRNOTAVAIL # IPv6 is not supported
-      exit
+      return
     end
+
     port = server.connect_address.ip_port
     server.close
 
@@ -353,7 +358,7 @@ def test_initialize_v6_connected_socket_with_v6_address
     begin
       server = TCPServer.new("::1", 0)
     rescue Errno::EADDRNOTAVAIL # IPv6 is not supported
-      exit
+      return
     end
 
     server_thread = Thread.new { server.accept }