tests: Simplify customization of %ENV

tests: Simplify customization of %ENV

Rather than having an %extraENV that is added to the default %ENV
and resetting %ENV _before_ each test, allow the test setup to
modify %ENV directly as needed then reset %ENV _after_ each test.

* tests/test_driver.pl: Remove unused %extraENV.
(resetENV): Don't add in %extraENV.
(_run_command): Reset after we run the command rather than before.
* tests/scripts/features/export: Convert %extraENV to %ENV
* tests/scripts/features/jobserver: Ditto
* tests/scripts/features/parallelism: Ditto
* tests/scripts/features/targetvars: Ditto
* tests/scripts/functions/eval: Ditto
* tests/scripts/functions/foreach: Ditto
* tests/scripts/functions/origin: Ditto
* tests/scripts/misc/general4: Ditto
* tests/scripts/options/dash-e: Ditto
* tests/scripts/targets/POSIX: Ditto
* tests/scripts/variables/GNUMAKEFLAGS: Ditto
* tests/scripts/variables/SHELL: Ditto