Manage environment variables used in Drush script in tests

In the past, we held off on such additions because there were already a couple of ways to set php options (php.ini or drush.ini in a location Drush searched for); however, I think that the ideal mechanism for setting php options is dependent on what you are trying to accomplish, and it is therefore valuable to be flexible in this area.

Therefore, for consideration, the attached patch allows the user to specify PHP_INI, DRUSH_INI or PHP_OPTIONS in environment variables, as desired.

Same patch, but with my debug 'echo' removed. :p

Committed #2, then added some documentation in 6b46616.

No tests yet.

Here is the beginning of some work to better manage environment variables in Drush tests and in backend invoke. This will allow us to test the Drush script, and to specify environment values to use with a site in its site alias.

#5 is finished enough for one test to work, so could be reviewed and committed if desired.

Pay no attention to that out-of-place $expected in the functional test.

There are failures in #5, such as:

6) coreCase::testRsyncPercentFiles
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-Calling system(rsync -e 'ssh ' -akz --yes --invoke /tmp/drush-sandbox/web/sites/dev/files /tmp);
+Calling system(rsync -e 'ssh ' -akz --yes --invoke @dev:'/tmp/drush-sandbox/web/%files' /tmp);

Looks like I dropped a var; will look at it later.

Any objection to back-porting #2 + #4 to 7.x-5.x dev? Still working on tests, but Jonhattan wants to role a 5.9 release. Remove 'Release blocker' tag if you don't want it to go in.

Committed 5ae596c, which allows tests to specify environment variables, and adds a test of the drush script using the PHP_OPTIONS environment variable. Environment variable handling in backend invoke and aliases can be done in a separate issue, as needed.

Is this (#2, #4, #10) desired for 7.x-5.x?

I noted that mistake in #7, but forgot to fix it. I'll remove the unused line and backport.

Backported to 7.x-5.x.