Move tests to separate directories, and document

Today, with the tests inside a `tests` intermingled with the
corresponding library's source code, we have a few problems:

- We have to be careful that wildcards don't end up with tests being
  built as part of Nix proper, or test headers being installed as part
  of Nix proper.

- Tests in libraries but not executables is not right:

  - It means each executable runs the previous unit tests again, because
    it needs the libraries.

  - It doesn't work right on Windows, which doesn't want you to load a
    DLL just for the side global variable . It could be made to work
    with the dlopen equivalent, but that's gross!

This reorg solves these problems.

There is a remaining problem which is that sibbling headers (like
`hash.hh` the test header vs `hash.hh` the main `libnixutil` header) end
up shadowing each other. This PR doesn't solve that. That is left as
future work for a future PR.

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

(cherry picked from commit 91b6833686a6a6d9eac7f3f66393ec89ef1d3b57)
(cherry picked from commit a61e42adb528b3d40ce43e07c79368d779a8b624)

1 files changed, 15 insertions, 0 deletions

diff --git a/tests/unit/libutil-support/local.mk b/tests/unit/libutil-support/local.mk
new file mode 100644
index 000000000..d085eda4e
--- /dev/null
+++ b/tests/unit/libutil-support/local.mk
@@ -0,0 +1,15 @@
+libraries += libutil-test-support
+
+libutil-test-support_NAME = libnixutil-test-support
+
+libutil-test-support_DIR := $(d)
+
+libutil-test-support_INSTALL_DIR :=
+
+libutil-test-support_SOURCES := $(wildcard $(d)/tests/*.cc)
+
+libutil-test-support_CXXFLAGS += $(libutil-tests_EXTRA_INCLUDES)
+
+libutil-test-support_LIBS = libutil
+
+libutil-test-support_LDFLAGS := -lrapidcheck