Update the testing harness to insulate tests in --nofork mode.

Before, when running a test binary in --nofork mode, it was up to the
test to reset the program state before exiting to avoid dependencies on
other tests. Now after each test the test harness will:

1. Wipeout the fake environmennt.
2. Reset the data segment to its initialization state.

This achieves reasonable insulation between tests even though certain
things (like a segfault) are stil not practical to completely insulate
without fork()'ing.

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