test: unflakify test cases (#4940)

With the recent unification of server and cmdline processing,
`IO.Process` tests that previously broke the server because they
directly wrote to stdout are now flaky on the cmdline because
elaboration and reporting are happening in separate threads. By removing
direct writes to stdout, the race condition is removed and the file can
actually be edited in the language server as well again.

tests/lean/Process.lean

@@ -7,13 +7,9 @@ def usingIO {α} (x : IO α) : IO α := x
   let child ← spawn { cmd := "sh", args := #["-c", "exit 1"] };
   child.wait
 
-#eval usingIO do
-  let child ← spawn { cmd := "sh", args := #["-c", "echo hi!"] };
-  child.wait
-
 #eval usingIO do
   let child ← spawn { cmd := "sh", args := #["-c", "echo ho!"], stdout := Stdio.piped };
-  discard $ child.wait;
+  child.wait >>= IO.println;
   child.stdout.readToEnd
 
 #eval usingIO do
@@ -53,10 +49,12 @@ def usingIO {α} (x : IO α) : IO α := x
     cmd := "lean",
     args := #["--stdin"]
     stdin := IO.Process.Stdio.piped
+    stdout := IO.Process.Stdio.piped
   }
   let (stdin, lean) ← lean.takeStdin
   stdin.putStr "#exit\n"
-  lean.wait
+  let _ ← lean.wait
+  lean.stdout.readToEnd
 
 #eval usingIO do
   let child ← spawn { cmd := "sh", args := #["-c", "cat"], stdin := .piped, stdout := .piped }

tests/lean/Process.lean.expected.out

@@ -1,5 +1,4 @@
 1
-hi!
 0
 "ho!\n"
 "hu!\n"
@@ -8,8 +7,7 @@ flush of broken pipe failed
 100000
 0
 0
-<stdin>:1:0: warning: using 'exit' to interrupt Lean
-0
 0
+"<stdin>:1:0: warning: using 'exit' to interrupt Lean\n"
 none
 some 0