Buildcache: bindist test without invoking spack compiler wrappers. (#15687)

* Buildcache:
   * Try mocking an install of quux, corge and garply using prebuilt binaries
   * Put patchelf install after ccache restore
   * Add script to install patchelf from source so it can be used on Ubuntu:Trusty which does not have a patchelf pat package. The script will skip building on macOS
   * Remove mirror at end of bindist test
   * Add patchelf to Ubuntu build env
   * Revert mock patchelf package to allow other tests to run.
   * Remove depends_on('patchelf', type='build') relying instead on
   * Test fixture to ensure patchelf is available.

* Call g++ command to build libraries directly during test build

* Flake8

* Install patchelf in before_install stage using apt unless on Trusty where a build is done.

* Add some symbolic links between packages

* Flake8

* Flake8:

* Update mock packages to write their own source files

* Create the stage because spec search does not create it any longer

* updates after change of list command arguments

* cleanup after merge

* flake8

share/spack/qa/install_patchelf.sh

@@ -0,0 +1,22 @@
+#!/bin/sh
+#
+# Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+#
+# Description:
+#     Install patchelf for use in buildcache unit tests
+#
+# Usage:
+#     install-patchelf.sh
+#
+set -ex
+if [ "$TRAVIS_OS_NAME" = "linux" ]; then
+    olddir=$PWD
+    cd /tmp
+    wget https://github.com/NixOS/patchelf/archive/0.10.tar.gz
+    tar -xvf 0.10.tar.gz
+    cd patchelf-0.10 && ./bootstrap.sh && ./configure --prefix=/usr && make && sudo make install && cd $olddir
+fi