[build] Force a fidl_library() with .test.fidl sources to be testonly

FIDL source files must be named `*.fidl` and if source files are
named `*.test.fidl` then the library using it can only be used in
test code.  So the fidl_library() template automatically sets
`testonly=true` if its sources list include any `*.test.fidl`.
It's not allowed to override this with explicit `testonly=false`.

Change-Id: I7bb79327be678efbfd293b9fa2baaad55832c9df