Add more fcntl and seal constants for Android/Linux

We now create an additional binary `linux_fcntl` for testing this
since there are header conflicts when including all necessary headers.
This binary is run on all platforms even though it's empty on all non-
Android/non-Linux platforms.

Testing has been switched from a custom binary to using a runner-less
test (or pair of tests). This means that for local development a simple
`cd libc-test && cargo test` will run all the tests. CI has also been
updated here to reflect that.
diff --git a/appveyor.yml b/appveyor.yml
index 22ef8a5..3121825 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -24,7 +24,7 @@
 
 test_script:
   - cargo test --target %TARGET%
-  - cargo run --manifest-path libc-test/Cargo.toml --target %TARGET%
+  - cargo test --manifest-path libc-test/Cargo.toml --target %TARGET%
 
 cache:
   - target