[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
needl...