Fix coverage for internal_repo_rocksdb

Summary:
tcc gtest runner need to know the location of the binary in order to collect coverage.  We can give them the location in an environment variable.

Note that all these tests will break in tpx currently, though this is a bug in rocksdb's wrapper script, not tpx.

Reviewed By: siying

Differential Revision: D20430043

fbshipit-source-id: c77d5f70bbc28f6011c6f91906bce2ceecc2f167

defs.bzl

@@ -35,8 +35,11 @@ def test_binary(
         external_deps = rocksdb_external_deps,
     )
 
+    binary_path = "$(location :{})".format(test_bin)
+
     custom_unittest(
         name = test_name,
-        command = [TEST_RUNNER, "$(location :{})".format(test_bin)],
+        command = [TEST_RUNNER, binary_path],
         type = ttype,
+        env = {"BUCK_BASE_BINARY": binary_path},
     )