FileWatching: Dump open file descriptors on failure

If the FDs we allocated are too high, we leaked fds somewhere.
On linux it's fairly easy to dump out information about all
active FDs, so do that in the failure case, so the logs can
help in debugging.

stdlib/FileWatching/test/runtests.jl

@@ -23,8 +23,14 @@ for i in 1:n
     end
     Ctype = Sys.iswindows() ? Ptr{Cvoid} : Cint
     FDmax = Sys.iswindows() ? 0x7fff : (n + 60) # expectations on reasonable values
-    @test 0 <= Int(Base.cconvert(Ctype, pipe_fds[i][1])) <= FDmax
-    @test 0 <= Int(Base.cconvert(Ctype, pipe_fds[i][2])) <= FDmax
+    fd_in_limits =
+        0 <= Int(Base.cconvert(Ctype, pipe_fds[i][1])) <= FDmax &&
+        0 <= Int(Base.cconvert(Ctype, pipe_fds[i][2])) <= FDmax
+    # Dump out what file descriptors are open for easier debugging of failure modes
+    if !fd_in_limits && Sys.islinux()
+        run(`ls -la /proc/$(getpid())/fd`)
+    end
+    @test fd_in_limits
 end
 
 function pfd_tst_reads(idx, intvl)