Fix nvm_ls_current fast unit test

[reasonablytall]

Fixes one of the failing tests in #2181

nvm_ls_current returns 'system' when nvm_tree_contains_path is aliased to return zero, as if nvm is deactivated. This fix instead sets NVM_DIR to match TEST_DIR so that nvm_tree_contains_path passes and the mock executable is actually used. Let me know if this approach is reasonable!

Relevant branches in nvm_ls_current:

nvm/nvm.sh

Lines 915 to 925 in e01060f

	elif nvm_tree_contains_path "${NVM_DIR}" "${NVM_LS_CURRENT_NODE_PATH}"; then
	local VERSION
	VERSION="$(node --version 2>/dev/null)"
	if [ "${VERSION}" = "v0.6.21-pre" ]; then
	nvm_echo 'v0.6.21'
	else
	nvm_echo "${VERSION}"
	fi
	else
	nvm_echo 'system'
	fi

[ljharb]

It's not clear to me why this test is passing in CI but fails locally, and I'm hesitant to change the test without knowing why this change fixes it.

[reasonablytall]

I think I mixed up return_zero being truthy/falsey in my initial explanation, and thought that that was the issue 😅

Digging deeper I believe I've figured out some things:

1. Inline aliasing nvm_tree_contains_path in the tests doesn't do anything because aliases don't apply until the next line (reference).
2. When running the tests via npm run test/fast, the which linked into the test directory is actually node_modules/.bin/which rather than the builtin.
   

   nvm/test/fast/Unit tests/nvm_ls_current

   Line 22 in e01060f

   ln -s "$(command which which)" "$TEST_DIR/which"

   
   That's a #!/usr/bin/env node script so when command which node 2>/dev/null is run in nvm_ls_current, the mock node is used, and NVM_LS_CURRENT_NODE_PATH is set to VERSION FOO!, which doesn't seem quite right 😆
   

   nvm/nvm.sh

   Line 911 in e01060f

   if ! NVM_LS_CURRENT_NODE_PATH="$(command which node 2>/dev/null)"; then

Based on this, I'm working on a fix that links the correct which during the test. Let me know if I'm missing something!

[ljharb]

Thanks, I really appreciate the digging, and this sounds like the right track.

[ljharb]

Thanks, almost there :-)

[ljharb]

i don't think /usr/bin will always be where it's located. i think the issue is that node_modules/.bin is in the PATH when npm run is used - so, can we remove that from the PATH rather than hardcoding one possible location of which?

[ljharb]

(－‸ლ) oops

[reasonablytall]

Thanks for the review! Made the change :)

[ljharb]

hmm, too bad there's no ergonomic way to use nvm_strip_path here