Add interface to get run and test deps

Since tests are identified as a single command, dependencies for tests
and run are implicitly assumed to be the same as for the Build step,
including the target binary. This is, however not correct because it
doesn't account for the fact that we may want to use  a different test
runner or some additional target that must be compiled before testing,
but not necessarily in the build step of the target.

With this purpose this change introduces an extendedTestInterface
and extendedRunInteface where the user may also specify a TestDeps() or
RunDeps() function to get the dependencies of the test/run step.

RULES/core/context.go

@@ -96,10 +96,24 @@ type runInterface interface {
 	Run(args []string) string
 }
 
+// An interface for runnables that depend on some other set of targets to run, but not to build
+// For example, when using a test wrapper.
+type extendedRunInterface interface {
+	runInterface
+	RunDeps() []OutPath
+}
+
 type testInterface interface {
 	Test(args []string) string
 }
 
+// An interface for tests that depend on some other set of targets to run, but not to build
+// For example, when using a test wrapper.
+type extendedTestInterface interface {
+	testInterface
+	TestDeps() []OutPath
+}
+
 type CoverageInterface interface {
 	Test(args []string) string
 	Binaries() []Path
@@ -314,9 +328,17 @@ func (ctx *context) handleTarget(targetPath string, target buildInterface) {
 	})
 
 	if runIface, ok := target.(runInterface); ok {
+		deps := []string{}
+		if extendedRunIface, ok := target.(extendedRunInterface); ok {
+			depsPaths := extendedRunIface.RunDeps()
+			for _, dep := range depsPaths {
+				deps = append(deps, dep.Absolute())
+			}
+		}
+
 		ctx.targetRules = append(ctx.targetRules, TargetRule{
 			Target: fmt.Sprintf("%s#run", targetPath),
-			Ins:    []string{targetPath},
+			Ins:    append(deps, targetPath),
 			Variables: map[string]string{
 				"command":     runIface.Run(input.RunArgs),
 				"description": fmt.Sprintf("Running %s:", targetPath),
@@ -326,9 +348,17 @@ func (ctx *context) handleTarget(targetPath string, target buildInterface) {
 	}
 
 	if testIface, ok := target.(testInterface); ok {
+		deps := []string{}
+		if extendedTestIface, ok := target.(extendedTestInterface); ok {
+			depsPaths := extendedTestIface.TestDeps()
+			for _, dep := range depsPaths {
+				deps = append(deps, dep.Absolute())
+			}
+		}
+
 		ctx.targetRules = append(ctx.targetRules, TargetRule{
 			Target: fmt.Sprintf("%s#test", targetPath),
-			Ins:    []string{targetPath},
+			Ins:    append(deps, targetPath),
 			Variables: map[string]string{
 				"command":     testIface.Test(input.TestArgs),
 				"description": fmt.Sprintf("Testing %s:", targetPath),