Fix testing with read-only source directory.

Three tests fail with a read-only source directory because they try to
write into the source directory.  None of these write into it in a way
that should actually be problematic for concurrent builds sharing the
same writable source directory, but avoiding any writing into the
source directory (from testing, or from building glibc if the source
timestamps are properly ordered) is still a good idea, as being able
to build with read-only sources helps make sure there isn't anything
that could cause problems for concurrent builds.

This patch changes the tests in question to use either /tmp or the
build directory to write their temporary files (or to test O_TMPFILE,
as applicable).

Tested for x86_64.

	* io/Makefile (tst-open-tmpfile-ARGS): New variable.
	* posix/tst-mmap-offset.c (fname): Use /tmp.
	* stdlib/tst-setcontext3.sh (tempfile): Use ${objpfx}.

ChangeLog

@@ -1,3 +1,9 @@
+2017-12-15 Joseph Myers <joseph@codesourcery.com>
+
+ * io/Makefile (tst-open-tmpfile-ARGS): New variable.
+ * posix/tst-mmap-offset.c (fname): Use /tmp.
+ * stdlib/tst-setcontext3.sh (tempfile): Use ${objpfx}.
+
 2017-12-15 Steve Ellcey <sellcey@cavium.com>
 
  * nscd/dbg_log.c (dbg_log): Increase msg buffer size.

io/Makefile

@@ -109,6 +109,8 @@ test-stat2-ARGS = Makefile . $(objpfx)test-stat2
 
 tst-statvfs-ARGS = $(objpfx)tst-statvfs tst-statvfs.c /tmp
 
+tst-open-tmpfile-ARGS = --test-dir=$(objpfx)
+
 ifeq ($(run-built-tests),yes)
 $(objpfx)ftwtest.out: ftwtest-sh $(objpfx)ftwtest
  $(SHELL) $< $(common-objpfx) '$(test-program-cmd)' > $@; \

posix/tst-mmap-offset.c

@@ -29,7 +29,7 @@
 
 static int fd;
 static long int page_shift;
-static char fname[] = "tst-mmap-offset-XXXXXX";
+static char fname[] = "/tmp/tst-mmap-offset-XXXXXX";
 
 static void
 do_prepare (int argc, char **argv)

stdlib/tst-setcontext3.sh

@@ -34,7 +34,7 @@ cleanup() {
 }
 trap cleanup 0
 
-tempfile=$(mktemp "tst-setcontext3.XXXXXXXXXX")
+tempfile=$(mktemp "${objpfx}tst-setcontext3.XXXXXXXXXX")
 
 # We want to run the test program and see if secontext called
 # exit() and wrote out the test file we specified. If the