ART: Fix oat_writer_test.

Use oat file names with .oat extension, so that
ReplaceFileExtension() used for opening the associated
vdex file does not replace everything starting from the
.temp folder.

Test: m test-art-host-gtest-oat_writer_test
Change-Id: I2fa9b3d2cca5054568777d7c05ce03f6eb1131b2