fix: make npm test portable to Windows

[Wuodan]

On Windows, the test command runs through two shell layers:

GitHub Actions (PowerShell)
-> npm cit
-> npm test (cmd.exe)
-> bats ${CI+-t} test

cmd.exe does not understand the POSIX ${CI+-t} expansion. It passes the expression to Bats as an argument, which Bats interprets as a test path and fails before running any tests.

The expansion was added in 2019 to select TAP output in CI, but Bats has done that automatically when CI is set since 2014. Remove the redundant expansion so npm test works across platforms without changing CI output.

This is related to PR 15 in bats-core/.github

[Wuodan]

Also related to PR 1258 fixing BATS_LIB_PATH on Windows in bats-core

Without it, this
BATS_LIB_PATH=$PWD/node_modules:${BATS_LIB_PATH-}
in

bats-assert/test/test_helper.bash

Line 2 in 697471b

BATS_LIB_PATH=$PWD/node_modules:${BATS_LIB_PATH-}

will fail on Windows runners.