Small refactoring changes

1) Move some provisioning unit tests into
   oemcrypto_provisioning_test.cpp
2) Remove some test group comments

Bug: 253779846
Merged from https://widevine-internal-review.googlesource.com/176735

Change-Id: I3d02c23b63eb7949fbc6abf60a7dbe32086ef2c9

libwvdrmengine/oemcrypto/test/oemcrypto_usage_table_test.cpp

@@ -10,9 +10,6 @@ using ::testing::Values;
 
 namespace wvoec {
 
-/// @addtogroup usage_table
-/// @{
-
 // Test that successive calls to PrepAndSignProvisioningRequest only increase
 // the provisioning count in the ODK message
 TEST_F(OEMCryptoSessionTests, Provisioning_IncrementCounterAPI18) {
@@ -1703,5 +1700,4 @@ INSTANTIATE_TEST_SUITE_P(TestAPI16, OEMCryptoUsageTableDefragTest,
 INSTANTIATE_TEST_SUITE_P(TestAPI16, OEMCryptoUsageTableTestWallClock,
                          Values<uint32_t>(kCurrentAPI));
 
-/// @}
 }  // namespace wvoec