build-system/r: Make test directory configurable.

Previously we would accept the #:test-directory argument but ignore the value.

* guix/build-system/r.scm (r-build): Set default value for TEST-DIRECTORY
field.
* guix/build/r-build-system.scm (patch-tests): Use value of TEST-DIRECTORY
argument.

Change-Id: If435577bc0e90ede360ee281ee5bc27fa1b0b3f0

guix/build-system/r.scm

@@ -110,7 +110,7 @@ release corresponding to NAME and VERSION."
                   (tests? #t)
                   (test-target "tests")
                   (test-types #f)
-                  test-directory
+                  (test-directory "tests/testthat")
                   skipped-tests
                   (configure-flags ''())
                   (phases '%standard-phases)

guix/build/r-build-system.scm

@@ -72,7 +72,7 @@
 each consisting of a file name under TEST-DIRECTORY and one or more names of
 tests to be skipped.  If the entry is just a file name, delete the test file."
   (when skipped-tests
-    (with-directory-excursion "tests/testthat/"
+    (with-directory-excursion test-directory
       (for-each (match-lambda
                   ((file test-names ...)
                    (substitute file