misc/cgo/testcshared: use file descriptor 30 for TestUnexportedSymbols

We were using file descriptor 100, which requires the Linux kernel to
grow the fdtable size. That step may sometimes require a long time,
causing the test to fail. Switch to file descriptor 30, which should
not require growing the fdtable.

Fixes #23784

Change-Id: I3ac40d6f8569c70d34b470cfca34eff149bf8229
Reviewed-on: https://go-review.googlesource.com/108537
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

misc/cgo/testcshared/main2.c

@@ -9,7 +9,7 @@
 #include <time.h>
 #include <unistd.h>
 
-#define fd (100)
+#define fd (30)
 
 // Tests libgo2.so, which does not export any functions.
 // Read a string from the file descriptor and print it.

misc/cgo/testcshared/src/libgo2/libgo2.go

@@ -21,7 +21,7 @@ import (
 // that the C code can also use.
 
 const (
-	fd = 100
+	fd = 30
 )
 
 func init() {