Add more vintf_object_test

* Fix duplicated Compat2 test in libvintf_test
The test is duplicated in libvintf_test and vintf_object_test;
it also fails because details::CheckCompatibility doesn't take
GetXCompatibilityMatrix functions.

* Add PartitionMounter that allows mocking on mount
procedures

* Fix duplicated mount and unmount in checkCompatibility

Test: libvintf_test
Test: vintf_object_test

Change-Id: I07be537696efe64f084fdbed77a781b26e971784
diff --git a/utils.cpp b/utils.cpp
index 3c4d2e7..9b20b01 100644
--- a/utils.cpp
+++ b/utils.cpp
@@ -20,9 +20,11 @@
 namespace vintf {
 namespace details {
 
-FileFetcher fetcher;
+static FileFetcher fetcher;
 FileFetcher* gFetcher = &fetcher;
 
+static PartitionMounter partitionMounter;
+PartitionMounter* gPartitionMounter = &partitionMounter;
 }  // namespace details
 }  // namespace vintf
 }  // namespace android