tests: fix misc cppcheck warnings

tests/driver_hdc1000/main.c

@@ -33,7 +33,6 @@ int main(void)
 {
     hdc1000_t dev;
     int16_t temp, hum;
-    size_t len;
     char tstr[8];
     char hstr[8];
 
@@ -49,6 +48,7 @@ int main(void)
     }
 
     while (1) {
+        size_t len;
         hdc1000_read(&dev, &temp, &hum);
 
         len = fmt_s16_dfp(tstr, temp, 2);

tests/driver_pn532/main.c

@@ -50,6 +50,8 @@ int main(void)
     ret = pn532_init_i2c(&pn532, &pn532_conf[0]);
 #elif defined(PN532_SUPPORT_SPI)
     ret = pn532_init_spi(&pn532, &pn532_conf[0]);
+#else
+#error None of PN532_SUPPORT_I2C and PN532_SUPPORT_SPI set!
 #endif
 
     if (ret != 0) {

tests/leds/main.c

@@ -77,6 +77,8 @@ int main(void)
 #endif
 
     puts("On-board LED test\n");
+    /* cppcheck-suppress knownConditionTrueFalse
+     * rationale: board-dependent ifdefs */
     if (numof == 0) {
         puts("NO LEDs AVAILABLE");
     }

tests/periph_hwrng/main.c

@@ -31,7 +31,7 @@ int main(void)
     uint8_t buf[LIMIT];
 
     puts("\nHWRNG peripheral driver test\n");
-    printf("This test will print from 1 to %i random bytes about every "
+    printf("This test will print from 1 to %u random bytes about every "
            "second\n\n", LIMIT);
 
     puts("Initializing the HWRNG driver.\n");

tests/pthread_cleanup/main.c

@@ -68,7 +68,7 @@ int main(void) {
     void *res;
     pthread_join(th_id, (void **) &res);
 
-    printf("Result: %u\n", (int) (intptr_t) res);
+    printf("Result: %i\n", (int) (intptr_t) res);
     puts("Done.");
     return 0;
 }

tests/unittests/tests-crypto/tests-crypto.h

@@ -40,7 +40,7 @@ void tests_crypto(void);
  */
 Test *tests_crypto_chacha_tests(void);
 
-static inline int compare(uint8_t a[16], uint8_t b[16], uint8_t len)
+static inline int compare(uint8_t *a, uint8_t *b, uint8_t len)
 {
     int result = 1;