Revert "Remove unit tests from old build system"

`make check` was reverted too soon. The hacking guide wasn't brought
up to date with the new workflow, and it's not clear how to use
meson for everything.

This reverts commit 6f3045c2a2.

tests/unit/libexpr/local.mk

@@ -0,0 +1,45 @@
+check: libexpr-tests_RUN
+
+programs += libexpr-tests
+
+libexpr-tests_NAME := libnixexpr-tests
+
+libexpr-tests_ENV := _NIX_TEST_UNIT_DATA=$(d)/data GTEST_OUTPUT=xml:$$testresults/libexpr-tests.xml
+
+libexpr-tests_DIR := $(d)
+
+ifeq ($(INSTALL_UNIT_TESTS), yes)
+  libexpr-tests_INSTALL_DIR := $(checkbindir)
+else
+  libexpr-tests_INSTALL_DIR :=
+endif
+
+libexpr-tests_SOURCES := \
+    $(wildcard $(d)/*.cc) \
+    $(wildcard $(d)/value/*.cc) \
+    $(wildcard $(d)/flake/*.cc)
+
+libexpr-tests_EXTRA_INCLUDES = \
+    -I tests/unit/libexpr-support \
+    -I tests/unit/libstore-support \
+    -I tests/unit/libutil-support \
+    $(INCLUDE_libexpr) \
+    $(INCLUDE_libexprc) \
+    $(INCLUDE_libfetchers) \
+    $(INCLUDE_libstore) \
+    $(INCLUDE_libstorec) \
+    $(INCLUDE_libutil) \
+    $(INCLUDE_libutilc)
+
+libexpr-tests_CXXFLAGS += $(libexpr-tests_EXTRA_INCLUDES)
+
+libexpr-tests_LIBS = \
+    libexpr-test-support libstore-test-support libutil-test-support \
+    libexpr libexprc libfetchers libstore libstorec libutil libutilc
+
+libexpr-tests_LDFLAGS := -lrapidcheck $(GTEST_LIBS) -lgmock
+
+ifdef HOST_WINDOWS
+  # Increase the default reserved stack size to 65 MB so Nix doesn't run out of space
+  libexpr-tests_LDFLAGS += -Wl,--stack,$(shell echo $$((65 * 1024 * 1024)))
+endif