Don't use libtest harness for filetests

[bjorn3]

We are using our own test harness for filetests and embedding it in libtest isn't useful. It only hides test output until the end and results in unnecessary noise.

Old failure output

    Finished test [unoptimized + debuginfo] target(s) in 0.11s
     Running tests/filetests.rs (/home/gh-bjorn3/wasmtime/target/debug/deps/filetests-1d6c16a18cf427e6)

running 1 test
test filetests ... FAILED

failures:

---- filetests stdout ----
thread 'worker #4' panicked at 'StructReturn return value not present!', cranelift/codegen/src/machinst/lower.rs:736:26
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread 'worker #10' panicked at 'StructReturn return value not present!', cranelift/codegen/src/machinst/lower.rs:736:26
FAIL filetests/filetests/isa/aarch64/call.clif: panicked in worker #10: StructReturn return value not present!
FAIL filetests/filetests/isa/x64/struct-ret.clif: panicked in worker #4: StructReturn return value not present!
444 tests
thread 'filetests' panicked at 'test harness: 2 failures', cranelift/tests/filetests.rs:5:10


failures:
    filetests

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 2.84s

error: test failed, to rerun pass '--test filetests'

New failure output

    Finished test [unoptimized + debuginfo] target(s) in 6.15s
     Running tests/filetests.rs (/home/gh-bjorn3/wasmtime/target/debug/deps/filetests-1d6c16a18cf427e6)
thread 'worker #31' panicked at 'StructReturn return value not present!', cranelift/codegen/src/machinst/lower.rs:736:26
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread 'worker #1' panicked at 'StructReturn return value not present!', cranelift/codegen/src/machinst/lower.rs:736:26
FAIL filetests/filetests/isa/aarch64/call.clif: panicked in worker #1: StructReturn return value not present!
FAIL filetests/filetests/isa/x64/struct-ret.clif: panicked in worker #31: StructReturn return value not present!
444 tests
Error: 2 failures
error: test failed, to rerun pass '--test filetests'