tests/stdin: refactor test application

The test application now prints in a loop the input character. In case
stdin is not ready yet after startup this lets the possibility to try to
send several time a character before failing.
The automatic test is now more robust on platforms where stdin takes
time before it gets in a ready state (some AVR, hifive).

tests/stdin/main.c

@@ -18,7 +18,10 @@
 
 int main(void)
 {
+    while (1) {
         int value = getchar();
         printf("You entered '%c'\n", (char)value);
+    }
+
     return 0;
 }

tests/stdin/tests/01-run.py

@@ -7,12 +7,25 @@
 # directory for more details.
 
 import sys
+import pexpect
 from testrunner import run
 
 
+TEST_INPUT = 'O'
+RETRIES = 5
+TIMEOUT = 3
+
+
 def testfunc(child):
-    child.sendline('O')
+    expected_output = 'You entered \'{}\''.format(TEST_INPUT)
-    child.expect_exact('You entered \'O\'')
+    for _ in range(0, RETRIES):
+        child.sendline(TEST_INPUT)
+        ret = child.expect_exact([expected_output, pexpect.TIMEOUT],
+                                 timeout=TIMEOUT)
+        if ret == 0:
+            break
+    else:
+        child.expect_exact(expected_output)
 
 
 if __name__ == "__main__":