[libcxx] [test] Prepend to PATH instead of overriding it

On Windows, the PATH env variable is used for locating dynamically
linked librarys, akin to LD_LIBRARY_PATH on Linux.

The tests that run with a dynamically linked libc++ used "--env
PATH=%{lib}" in the test config. This had the unfortunate side effect
of making other tools from PATH unavailable during the runtime of the
tests; in particular, it caused the "executor-has-no-bash" flag to be
set for all those Windows test configs (with the clang-cl static config
being the only one lacking it).

Thus, this increases the number of tests actually included in the
clang-cl dll and all mingw test configs by 9 tests.

The clang-cl static test configuration has been executing those tests
since the "--env PATH=%{lib}" was removed from that test config in
e78223e7. (For mingw we haven't had a
need to split the test config between shared and static, which means
that the mingw static test config previously ran with --env PATH
needlessly.)

This increases the test coverage for patches like D146398 which
can't be executed in the executor-has-no-bash configs.

Change the default value of the arg.env to an empty array; when we do
pass values to the option, they get passed as an array of strings,
so make sure the variable behaves consistently when no arguments
have been passed.

Differential Revision: https://reviews.llvm.org/D148324