Add external-tests scripts

We keep test-cases using binaries rather than sources in a seperate test
directory dwz-external.tests.  The directory currently has a size of ~512MB.

This directory can be installed using the scripts in this patch:
...
$ ./scripts/fetch-external-tests.sh
$ ./scripts/untar-external-tests.sh
$ ls testsuite
dwz-external.tests  dwz.tests  lib  scripts
...

Result with current (md5sum 09cd309b8e720242cfc79d8de75c8563)
dwz-external-tests.tgz:
...
$ cat dwz.sum
Test run by vries on Wed Mar  6 15:56:13 2019
Native configuration is x86_64-pc-linux-gnu

                === dwz tests ===

Schedule of variations:
    unix

Running target unix
Running testsuite/dwz-external.tests/dwz-external-tests.exp ...
FAIL: testsuite/dwz-external.tests/pr24169.sh
FAIL: testsuite/dwz-external.tests/pr24171.sh
FAIL: testsuite/dwz-external.tests/pr24170.sh
FAIL: testsuite/dwz-external.tests/pr24172.sh
FAIL: testsuite/dwz-external.tests/pr24173.sh
FAIL: testsuite/dwz-external.tests/pr24195.sh
FAIL: testsuite/dwz-external.tests/pr24204.sh
Running testsuite/dwz.tests/dwz-tests.exp ...
PASS: testsuite/dwz.tests/low-mem.sh
PASS: testsuite/dwz.tests/multifile.sh
PASS: testsuite/dwz.tests/regular.sh
PASS: testsuite/dwz.tests/too-many-dies.sh
PASS: testsuite/dwz.tests/hardlink.sh
PASS: testsuite/dwz.tests/multifile-name.sh
PASS: testsuite/dwz.tests/multifile-relative.sh
PASS: testsuite/dwz.tests/regular-o.sh

                === dwz Summary ===

nr of expected passes            8
nr of unexpected failures        7
...

---
 scripts/fetch-external-tests.sh | 5 +++++
 scripts/tar-external-tests.sh   | 6 ++++++
 scripts/untar-external-tests.sh | 5 +++++
 3 files changed, 16 insertions(+)

diff --git a/scripts/fetch-external-tests.sh b/scripts/fetch-external-tests.sh
new file mode 100755
index 0000000..6a521ef
--- /dev/null
+++ b/scripts/fetch-external-tests.sh
@@ -0,0 +1,5 @@
+#!/bin/sh
+
+wget \
+    http://ftp.suse.com/pub/people/tdevries/dwz/dwz-external-tests.tgz
+
diff --git a/scripts/tar-external-tests.sh b/scripts/tar-external-tests.sh
new file mode 100755
index 0000000..9fe87fa
--- /dev/null
+++ b/scripts/tar-external-tests.sh
@@ -0,0 +1,6 @@
+#!/bin/sh
+
+tar \
+    cvfz \
+    ../dwz-external-tests.tgz \
+    testsuite/dwz-external.tests
diff --git a/scripts/untar-external-tests.sh b/scripts/untar-external-tests.sh
new file mode 100755
index 0000000..7193725
--- /dev/null
+++ b/scripts/untar-external-tests.sh
@@ -0,0 +1,5 @@
+#!/bin/sh
+
+tar \
+    xvfz \
+    dwz-external-tests.tgz