tests: Make builds less expensive.

tests: Make builds less expensive.

The switch to the reduced bootstrap broke build time assumptions made by
tests, notably the assumption that GNU-MAKE-BOOT0 was cheap to build.
This commit adjusts this to make these tests cheaper.

* gnu/packages/bootstrap.scm (%bootstrap-inputs-for-tests): New variable.
* guix/tests.scm (gnu-make-for-tests): New variable.
* tests/guix-environment.sh: Use GNU-MAKE-FOR-TESTS instead of
GNU-MAKE-BOOT0.  Remove test with FINDUTILS-BOOT0.
* tests/guix-package-net.sh (boot_make): Use GNU-MAKE-FOR-TESTS.
* tests/packages.scm ("GNU Make, bootstrap"): Likewise.
* tests/profiles.scm ("profile-derivation relative symlinks, two entries"):
Likewise.
* tests/union.scm (%bootstrap-inputs): Remove.
("union-build"): Use %BOOTSTRAP-INPUTS-FOR-TESTS instead of
%BOOTSTRAP-INPUTS.