Add Config,RunningInsideUnitTest

Sometimes it is necessary for some functionality to be disabled while
running in unit tests, e.g. functionality that requires external
information such as error prone tools and configuration. Sometimes it
is necessary for some functionality to be enabled while running in
unit tests, e.g. functionality that makes state available for
testing but which is not necessary at runtime.

Previously, that was done by checking to see if TestProductVariables
was nil. This change adds a method to abstract that.

Bug: 245956352
Test: m nothing
Change-Id: I7845b79328e7180623161a9bf897568089da4e4f

java/base.go

@@ -893,7 +893,7 @@ func (j *Module) collectBuilderFlags(ctx android.ModuleContext, deps deps) javaB
 
 	epEnabled := j.properties.Errorprone.Enabled
 	if (ctx.Config().RunErrorProne() && epEnabled == nil) || Bool(epEnabled) {
-		if config.ErrorProneClasspath == nil && ctx.Config().TestProductVariables == nil {
+		if config.ErrorProneClasspath == nil && !ctx.Config().RunningInsideUnitTest() {
 			ctx.ModuleErrorf("cannot build with Error Prone, missing external/error_prone?")
 		}