Fix config tests when environment variables are set

[cs278]

Composer reads environment variables to overload config variables but the tests do not expect this, so the following incantation fails:

$ COMPOSER_CACHE_DIR=/tmp vendor/bin/phpunit --stop-on-failure

PHPUnit 3.7.37 by Sebastian Bergmann.

Configuration read from /home/chris.smith/src/composer/phpunit.xml.dist

.............................................................   61 / 1245 (  4%)
..................................F

Time: 1.99 seconds, Memory: 25.75Mb

There was 1 failure:

1) Composer\Test\ConfigTest::testVarReplacement
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'/home/chris.smith/foo'
+'/tmp'

/home/chris.smith/src/composer/tests/Composer/Test/ConfigTest.php:121

FAILURES!
Tests: 96, Assertions: 362, Failures: 1.

This PR actually has two resolutions implemented, see 725a4fd. I think the second implementation makes more sense, I will remove the first two commits out if that's the consensus.

[stof]

IMO, resetting the environment in the test to ensure it is clean is a better solution than adding an option in the code to be able to ignore the environment

[cs278]

IMO, resetting the environment in the test to ensure it is clean is a better solution than adding an option in the code to be able to ignore the environment

I was following that thinking initially, but then considered that disabling the access of global state would was a better option and could potentially be reused elsewhere.

[naderman]

Indeed, who knows what else this may end up being useful for and it's not like it's a huge amount of code to enable this.