testsuite: fix subprocess fd leak check

Problem: several subprocess unit tests check for file descriptor
leaks but don't account for file descriptors opened by the
reactor itself at runtime, such as for signalfd(2).

Move fd sampling period to enclose reactor creation/destruction.

Author: garlick

src/common/libsubprocess/test/channel.c

@@ -475,12 +475,12 @@ int main (int argc, char *argv[])
 
     plan (NO_PLAN);
 
+    start_fdcount = fdcount ();
+
     // Create shared reactor for all tests
     ok ((r = flux_reactor_create (FLUX_REACTOR_SIGCHLD)) != NULL,
         "flux_reactor_create");
 
-    start_fdcount = fdcount ();
-
     diag ("channel_fd_env");
     test_channel_fd_env (r);
     diag ("channel_fd_in");
@@ -494,12 +494,13 @@ int main (int argc, char *argv[])
     diag ("bufsize_error");
     test_bufsize_error (r);
 
+    flux_reactor_destroy (r);
+
     end_fdcount = fdcount ();
 
     ok (start_fdcount == end_fdcount,
         "no file descriptors leaked");
 
-    flux_reactor_destroy (r);
     done_testing ();
     return 0;
 }

src/common/libsubprocess/test/stdio.c

@@ -1596,12 +1596,12 @@ int main (int argc, char *argv[])
 
     plan (NO_PLAN);
 
+    start_fdcount = fdcount ();
+
     // Create shared reactor for all tests
     ok ((r = flux_reactor_create (FLUX_REACTOR_SIGCHLD)) != NULL,
         "flux_reactor_create");
 
-    start_fdcount = fdcount ();
-
     diag ("basic_stdout");
     test_basic_stdout (r);
     diag ("basic_stdout_no_readline");
@@ -1655,12 +1655,13 @@ int main (int argc, char *argv[])
     diag ("on_credit_borrow_credits");
     test_on_credit_borrow_credits (r);
 
+    flux_reactor_destroy (r);
+
     end_fdcount = fdcount ();
 
     ok (start_fdcount == end_fdcount,
         "no file descriptors leaked");
 
-    flux_reactor_destroy (r);
     done_testing ();
     return 0;
 }

src/common/libsubprocess/test/subprocess.c

@@ -1142,12 +1142,12 @@ int main (int argc, char *argv[])
 
     plan (NO_PLAN);
 
+    start_fdcount = fdcount ();
+
     // Create shared reactor for all tests
     ok ((r = flux_reactor_create (FLUX_REACTOR_SIGCHLD)) != NULL,
         "flux_reactor_create");
 
-    start_fdcount = fdcount ();
-
     diag ("corner_cases");
     test_corner_cases (r);
     diag ("post_exec_errors");
@@ -1196,12 +1196,13 @@ int main (int argc, char *argv[])
     diag ("test_fdcleanup posix-spawn");
     test_fdcleanup (r, "posix-spawn", 0, 3);
 
+    flux_reactor_destroy (r);
+
     end_fdcount = fdcount ();
 
     ok (start_fdcount == end_fdcount,
         "no file descriptors leaked");
 
-    flux_reactor_destroy (r);
     done_testing ();
     return 0;
 }