Use the newly fixed test -t in isatty

Removes a weird dependency on the test command.

share/functions/isatty.fish

@@ -25,9 +25,5 @@ function isatty -d "Tests if a file descriptor is a tty"
             set fd $argv[1]
     end
 
-    # Use `command test` because `builtin test` doesn't open the regular fd's.
-    # See https://github.com/fish-shell/fish-shell/issues/1228
-    # Too often `command test` is some bogus Go binary, I don't know why. Use [ because
-    # it's less likely to be something surprising. See #5665
-    command [ -t "$fd" ]
+    test -t "$fd"
 end

tests/pexpects/isatty.py

@@ -57,3 +57,11 @@ sendline("t </dev/null")
 expect_str("stdout")
 expect_str("stderr")
 expect_prompt()
+
+sendline("isatty stdin && echo yes")
+expect_str("yes")
+expect_prompt()
+
+sendline("cat </dev/null | isatty stdin || echo no")
+expect_str("no")
+expect_prompt()