[Fix] nvm exec: Do a version check on nvm-exec

[userwiths]

This check would display a message in case the .nvmrc version is not installed, and would not alter the output otherwise.

nvm_ensure_version_installed returns the message that we care for.
It requires an argument (version) against which to do the check, which we get after calling nvm_rc_version

Fixes #1769

[ljharb]

This would need tests

[userwiths]

A test case was added that checks if the message printed out contains the relevant information (different version of node).

[ljharb]

Thanks! (I'm going to tweak the test to be "fast" instead of "slow", and to use a "never can exist" version instead of needing to remove a version, but otherwise this is great!)

[ljharb]

Unfortunately there's 10 tests failing. It's possible it's just that the expected output needs to be updated to reflect the change, but I might not have time to work on it soon, so it'd be great if you could take a look :-)

[userwiths]

Gave it a quick look.
The issues that came from my test were related to me forgetting to remove the .nvmrc that I create during the test.

Also saw that the version color printing test was failing, and went ahead and updated it.

Currently staying on 7 errors when running in the docker container, but all other failures seem unrelated to the current changes and a bit more time consuming to debug.

[ljharb]

aha, good catch. note tho that this doesn't actually restore what might already be present - we should back up and restore the existing .nvmrc in setup/teardown files

i'll take a crack at this part tho

[ljharb]

k, rebased this, and updated nvm-exec so it only calls nvm_rc_version when NODE_VERSION is not provided (when it is provided, nvm use ensures it's installed already)