Integration tests with ARGS or ENV in their add_integration_test() call
were being run with no arguments or environment variables by the
LibcTest lit format, causing failures in startup_args_test,
sprintf_size_test, getenv_test, execv_test, execve_test, and
getcwd_test.
Fix this by having add_integration_test() emit a sidecar
<executable>.params JSON file (via file(GENERATE)) whenever ARGS or ENV
are specified. LibcTest.execute() reads this file at test time and
passes the args and environment to the test executable.
Also always set PWD=exec_dir in the test environment so that
getenv("PWD") matches getcwd() for tests that compare them.
31f69d333e