[sanitizer_common] Add test for popen()

[krytarowski]

It looks fine, but we don't have interceptors for popen(3), popenve(3), pclose(3). Could you include them together with this patch? Add add a test for popenve(3).

[krytarowski]

But of course new code can be added as a new revision.

[mgorny]

Added assertion verifying that fileno(fp) works.

test/sanitizer_common/TestCases/Posix/popen.cc

+// RUN: %clangxx -g %s -o %t && %run %t | FileCheck %s
+// CHECK: 1
+// CHECK-NEXT: 2
+
+#include <assert.h>
+#include <stdio.h>
+
+int main(void) {
+  // use a tool that produces different output than input to verify
+  // that everything worked correctly
+  FILE *fp = popen("sort", "w");
+  assert(fp);
+
+  // verify that fileno() returns a meaningful descriptor (needed
+  // for the implementation of TSan)
+  assert(fileno(fp) != -1);
+
+  assert(fputs("2\n", fp) >= 0);
+  assert(fputs("1\n", fp) >= 0);
+  assert(pclose(fp) == 0);
+
+  return 0;
+}