Increase the test coverage of why-depends

- Test with `--derivation`
- Actually test with ca-derivations (was suuposedly done, but not
  activated because of a missing line in `local.mk`)

(cherry picked from commit 6a90ef072c)

tests/local.mk

@@ -90,6 +90,7 @@ nix_tests = \
   fmt.sh \
   eval-store.sh \
   why-depends.sh \
+  ca/why-depends.sh \
   import-derivation.sh \
   ca/import-derivation.sh \
   nix_path.sh \

tests/why-depends.sh

@@ -6,6 +6,9 @@ cp ./dependencies.nix ./dependencies.builder0.sh ./config.nix $TEST_HOME
 
 cd $TEST_HOME
 
+nix why-depends --derivation --file ./dependencies.nix input2_drv input1_drv
+nix why-depends --file ./dependencies.nix input2_drv input1_drv
+
 nix-build ./dependencies.nix -A input0_drv -o dep
 nix-build ./dependencies.nix -o toplevel