Use alias for test_per_src apex dependencies

AddFarVariationDependencies was broken, which allowed apex to add
dependencies on tests by requesting an empty test_per_src variation
even though some test dependencies did not have a test_per_src
variation.  Add an alias from the pre-test_per_src variation
variant to the empty test_per_src variation, and drop the
test_per_src variation when requesting the depencency.

Test: TestApexWithTests
Change-Id: I2ed7bdd761027956141e25d4d81f9074afe628b6
diff --git a/cc/test.go b/cc/test.go
index ee103ff..619dc4d 100644
--- a/cc/test.go
+++ b/cc/test.go
@@ -223,6 +223,7 @@
 					tests[i].(*Module).linker.(testPerSrc).setSrc(testNames[i], src)
 					mctx.AddInterVariantDependency(testPerSrcDepTag, all_tests, tests[i])
 				}
+				mctx.AliasVariation("")
 			}
 		}
 	}