Include a check for getentropy interception

This snippet applies to both the library constructors and variable
data test frameworks.

test/snippets/getentropy.c

@@ -0,0 +1,6 @@
+unsigned int targ;
+if (getentropy(&targ, sizeof(targ)) == 0) {
+  printf("[%s] getentropy() yielded 0x%08x\n", where, targ);
+} else {
+  printf("[%s] getentropy() failed with %d (%s)\n", where, errno, strerror(errno));
+}

test/snippets/getentropy.variable

@@ -0,0 +1 @@
+FAKERANDOM_SEED 0x0123456789ABCDEF

test/snippets/include_headers.h

@@ -3,3 +3,5 @@
 #include <unistd.h>
 #include <sys/random.h>
 #include <sys/syscall.h>
+#include <string.h>
+#include <errno.h>