testserver: respect the TMPDIR environment variable

This will be useful for cockroachdb CI, which has logic that can
preserve temporary files in TMPDIR in the CI artifacts.

It's safe to remove the logic to use "/tmp" since that was initially
added in a2ed1c6, but it's no longer needed since the testserver no
longer creates UNIX sockets.

testserver/testserver.go

@@ -455,12 +455,9 @@ func NewTestServer(opts ...TestServerOpt) (TestServer, error) {
 		}
 	}
 
-	// Force "/tmp/" so avoid OSX's really long temp directory names
-	// which get us over the socket filename length limit.
-	baseDir, err := os.MkdirTemp("/tmp", "cockroach-testserver")
+	baseDir, err := os.MkdirTemp("", "cockroach-testserver")
 	if err != nil {
-		return nil, fmt.Errorf("%s: could not create temp directory: %w",
-			testserverMessagePrefix, err)
+		return nil, fmt.Errorf("%s: could not create temp directory: %w", testserverMessagePrefix, err)
 	}
 
 	mkDir := func(name string) (string, error) {