tests/bloom_bytes: replace %f with fmt/print_float

Even if using `%f` the `printf_float` module was not used.

When running the test on `samr21-xpro` and `arduino-mega2560` the float
is not printed correctly.

 * samr21-xpro ` false positive rate.`
 * arduino-mega2560 `? false positive rate.`

As the arduino-mega2560 does not handle printf_float use
`fmt/print_float`.

The output should be flushed before using fmt/print functions if
available as they do not use `printf` buffer.

tests/bloom_bytes/Makefile

@@ -8,6 +8,8 @@ USEMODULE += bloom
 USEMODULE += random
 USEMODULE += xtimer
 
+USEMODULE += fmt
+
 DISABLE_MODULE += auto_init
 
 TEST_ON_CI_WHITELIST += all

tests/bloom_bytes/main.c

@@ -23,6 +23,7 @@
 #include <inttypes.h>
 
 #include "xtimer.h"
+#include "fmt.h"
 
 #include "hashes.h"
 #include "bloom.h"
@@ -109,7 +110,14 @@ int main(void)
     printf("%d elements probably in the filter.\n", in);
     printf("%d elements not in the filter.\n", not_in);
     double false_positive_rate = (double) in / (double) lenA;
-    printf("%f false positive rate.\n", false_positive_rate);
+    /* Use 'fmt/print_float' to work on all platforms (atmega)
+     * Stdout should be flushed before to prevent garbled output. */
+#ifdef MODULE_NEWLIB
+    /* no fflush on msp430 */
+    fflush(stdout);
+#endif
+    print_float(false_positive_rate, 6);
+    puts(" false positive rate.");
 
     bloom_del(&bloom);
     printf("\nAll done!\n");