Do not enable global ThinLTO for tests

Tests are not shipped, no need to spend extra CPU cycles optimising them
during build.

Test: GLOBAL_THINLTO=true m
Bug: 169004486
Change-Id: I66ede9c01d43b574a7fe9f74f0bc6ba97f51be06

cc/cc.go

@@ -485,6 +485,7 @@ type ModuleContextIntf interface {
 	static() bool
 	staticBinary() bool
 	testBinary() bool
+	testLibrary() bool
 	header() bool
 	binary() bool
 	object() bool
@@ -1486,6 +1487,10 @@ func (ctx *moduleContextImpl) testBinary() bool {
 	return ctx.mod.testBinary()
 }
 
+func (ctx *moduleContextImpl) testLibrary() bool {
+	return ctx.mod.testLibrary()
+}
+
 func (ctx *moduleContextImpl) header() bool {
 	return ctx.mod.Header()
 }