Remove "--full-shutdown" and instead use an environment variable LLD_IN_TEST.

We are running lld tests with "--full-shutdown" option because we don't
want to call _exit() in lld if it is running tests. Regular shutdown
is needed for leak sanitizer.

This patch changes the way how we tell lld that it is running tests.
Now "--full-shutdown" is removed, and LLD_IN_TEST environment variable
is used instead.

This patch enables full shutdown on all ports, e.g. ELF, COFF and wasm.
Previously, we enabled it only for ELF.

Differential Revision: https://reviews.llvm.org/D43410

llvm-svn: 325413