Add feature flags to test files.

Cranelift can be compiled with feature flags which can change its output. To
accomodate changes of output related to feature flags, test file can now include
`feature "..."` and `feature ! "..."` directives in the preamble of the test
file.

The test runner would skip the test if the flag does not match the expectation
of the test case.

cranelift/filetests/Cargo.toml

@@ -20,3 +20,6 @@ log = "0.4.6"
 memmap = "0.7.0"
 num_cpus = "1.8.0"
 region = "2.1.2"
+
+[features]
+basic-blocks = []