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

rust/rust.go

@@ -39,7 +39,6 @@ func init() {
 	android.RegisterModuleType("rust_defaults", defaultsFactory)
 	android.PreDepsMutators(func(ctx android.RegisterMutatorsContext) {
 		ctx.BottomUp("rust_libraries", LibraryMutator).Parallel()
-		ctx.BottomUp("rust_unit_tests", TestPerSrcMutator).Parallel()
 		ctx.BottomUp("rust_begin", BeginMutator).Parallel()
 	})
 	pctx.Import("android/soong/rust/config")