Add exported_bootclasspath_fragments to prebuilt_apex/apex_set

This is needed to allow a prebuilt_bootclasspath_fragment to be used
interchangeably with a bootclasspath_fragment in the
platform_bootclasspath module.

The platform_bootclasspath module depends on APEX specific variants of
bootclasspath_fragment modules. That works because the
bootclasspath_fragment modules are part of an apex and so have an APEX
specific variant which the platform_bootclasspath can specify.

Using a prebuilt_bootclasspath_fragment in place of a
bootclasspath_fragment requires that the prebuilt also has an APEX
specific variant.

Specifying exported_bootclasspath_fragments on a prebuilt_apex/apex_set
will cause it to create an APEX variant for the named module whcih will
allow it to be selected by the platform_bootclasspath module.

Bug: 186034565
Bug: 177892522
Test: m nothing
Change-Id: I7ddacc6498ec3a4a9f26c5f78b7f9a033e494d78
diff --git a/sdk/testing.go b/sdk/testing.go
index bf59aed..f4e85c0 100644
--- a/sdk/testing.go
+++ b/sdk/testing.go
@@ -254,6 +254,7 @@
 	snapshotPreparer := android.GroupFixturePreparers(sourcePreparers, fs.AddToFixture())
 
 	var runSnapshotTestWithCheckers = func(t *testing.T, testConfig snapshotTest, extraPreparer android.FixturePreparer) {
+		t.Helper()
 		customization := snapshotBuildInfo.snapshotTestCustomization(testConfig)
 		customizedPreparers := android.GroupFixturePreparers(customization.preparers...)