Make testpilot recognize that these tests have coverage instrumentation

Summary: TestPilot uses two flags to determine whether coverage is already instrumented: `fbcode_macros` and `coverage`.  Normally, these two tags are added automatically to cpp tests, but this is a fake cpp test, so we must manually add them.  The first is easy - `fbcode_macros` is added by the `custom_unittest` library, which is in `fbcode_macros`, so it is appropriate.  The second is harder - we need to verify that we should add the macro.  We do this using the `coverage.bzl` functions.

Reviewed By: siying

Differential Revision: D20549040

fbshipit-source-id: d2732b3ec26f3dff065efdf398abe3241075bb2f

defs.bzl

@@ -3,6 +3,7 @@
 # defs.bzl - Definitions for Facebook-specific buck build integration
 # in TARGETS
 
+load("@fbcode_macros//build_defs:coverage.bzl", "coverage")
 load("@fbcode_macros//build_defs:cpp_binary.bzl", "cpp_binary")
 load("@fbcode_macros//build_defs:custom_unittest.bzl", "custom_unittest")
 
@@ -37,9 +38,19 @@ def test_binary(
 
     binary_path = "$(location :{})".format(test_bin)
 
+    base_path = native.package_name()
+    tags = []
+    if coverage.is_coverage_enabled(base_path):
+        # This tag instructs testpilot to use
+        # the lower-memory coverage runner
+        # (e.g. it tells testpilot that the binary
+        # is actually instrumented with coverage info)
+        tags = ["coverage"]
+
     custom_unittest(
         name = test_name,
         command = [TEST_RUNNER, binary_path],
         type = ttype,
         env = {"BUCK_BASE_BINARY": binary_path},
+        tags = tags,
     )