Make rust_test file output more similar to cc_test.

This changes the way the output filename is calculated for rust_test
binaries to be more similar to cc_test.

This also removes the option to define multiple test binaries in a
single rust_test module via the TestPerSrc mutator. Now each rust_test
module corresponds to a single test binary.

Bug: 158500462
Test: m -j pin-utils_tests_pin_utils
Test: m -j unicode-xid_device_tests_unicode_xid
Change-Id: I6e0f79dcb4e49fa49d6ebb36abeef67a9eb180a0