tests/periph_rtt_min: harden test

- Better self-containment: read number of expected samples from output
  rather than the environment
- Less reliance on `stdout` timing: Count samples in application and
  output it rather then counting flushed dots.

tests/periph_rtt_min/Makefile

@@ -23,5 +23,3 @@ else
   SAMPLES ?= 1024
 endif
 CFLAGS += -DSAMPLES=$(SAMPLES)
-
-$(call target-export-variables, test, SAMPLES)

tests/periph_rtt_min/main.c

@@ -44,6 +44,7 @@ void cb(void *arg)
 int main(void)
 {
     uint32_t value = 0;
+    uint32_t samples = 0;
     /* mutex starts out locked, and each time an rtt callback is successfully
        called it will be locked again for the next iteration */
     mutex_t lock = MUTEX_INIT_LOCKED;
@@ -69,10 +70,12 @@ int main(void)
         }
         printf(".");
         fflush(stdout);
+        samples++;
     }
     printf("\n");
 
-    printf("RTT_MIN_OFFSET for %s: %" PRIu32 "\n", RIOT_BOARD, value);
+    printf("RTT_MIN_OFFSET for %s over %" PRIu32 " samples: %" PRIu32 "\n",
+           RIOT_BOARD, samples, value);
 
     return 0;
 }

tests/periph_rtt_min/tests/01-run.py

@@ -6,18 +6,19 @@
 # General Public License v2.1. See the file LICENSE in the top level
 # directory for more details.
 
-import os
 import sys
 
 from testrunner import run
 
-SAMPLES = int(os.getenv("SAMPLES", "1024"))
-
 
 def testfunc(child):
-    for _ in range(0, SAMPLES):
-        child.expect_exact('.')
-    child.expect(r'RTT_MIN_OFFSET for [a-zA-Z\-\_0-9]+: \d+')
+    child.expect(r"Evaluate RTT_MIN_OFFSET over (\d+) samples")
+
+    exp_samples = int(child.match.group(1))
+    child.expect(
+        r'RTT_MIN_OFFSET for [a-zA-Z\-\_0-9]+ over {samples} samples: \d+'
+        .format(samples=exp_samples)
+    )
 
 
 if __name__ == "__main__":