Bluetooth: Move re-implementation of snprintk

Move the function in the `subsys/testsuite/ztest/src/ztest_mock.c` files.

This is motivated by the fact that there is others re-implementation of
`*printk` functions using libc counterparts in the `ztest_mock.c` file.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>

subsys/testsuite/ztest/src/ztest_mock.c

@@ -56,6 +56,19 @@ void vprintk(const char *fmt, va_list ap)
 {
 	vprintf(fmt, ap);
 }
+
+int snprintk(char *str, size_t size, const char *fmt, ...)
+{
+	va_list ap;
+	int ret;
+
+	va_start(ap, fmt);
+	ret = snprintf(str, size, fmt, ap);
+	va_end(ap);
+
+	return ret;
+}
+
 #else
 
 /*