test: only build requested unit tests in nix

[lrstewart]

Description of changes:

Fixes a couple usability issues with the nix develop script that were bugging me:

• Functions should fail on the first error, not try to continue. Putting "-e" at the top of the script causes the whole shell to exit, but using subshells seems to work.
• If I'm just trying to run one unit test, I don't want to build the resources needed by the integration tests. Especially not s2n_head.
• If I'm just trying to run one unit test, I don't want to rebuild all the unit tests.
• If I'm trying to run unit tests, I probably also want to recompile. It's a much rarer use case to just want to retry a test with no changes. And if that is the case, the recompile should basically be a no-op.

Testing:

I ran variations of the command locally. It seems to be able to handle wildcards / exact names / nothing just fine. The CI has a test of the nothing / all tests case.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[dougch]

Makes sense; easier to iterate

[lrstewart]

I shouldn't have just automatically stuck every command in a subshell 😅

• "apache2_config" is only called by "apache2_start" to export a bunch of variables that "apache2_start" needs. Exporting those variables inside a subshell doesn't make them available to the parent, causing the nix integ tests to fail to build. I fixed that by removing the subshell for "apache2_config".
• "apache2_start" is called by "integ" to start the apache server. But the server only lives as long as the subshell. I fixed that by removing the subshell for "apache2_start" too.

I also checked the other functions for similar problems: no other exports, and no other private / helper functions.

Out of paranoia, I also double-checked that exports are available in commands as expected:

[nix openssl-3.0] ubuntu@ip-172-31-28-113:~/s2n-tls$ export S2N_CMAKE_OPTIONS=notarealoption
[nix openssl-3.0] ubuntu@ip-172-31-28-113:~/s2n-tls$ echo $S2N_CMAKE_OPTIONS
notarealoption
[nix openssl-3.0] ubuntu@ip-172-31-28-113:~/s2n-tls$ configure
+---------------------------------------------------------+
| Configuring with cmake                                  |
+---------------------------------------------------------+
CMake Warning:
  Ignoring extra path from command line:

   "notarealoption"

[dougch]

renegotiate_apache passes, LGTM