Load all fixture grammars dynamically

This way the build doesn't take forever any time a single grammar has 
been regenerated.

cli/src/tests/fixtures.rs

@@ -0,0 +1,51 @@
+use crate::loader::Loader;
+use std::path::{Path, PathBuf};
+use tree_sitter::Language;
+use std::fs;
+
+lazy_static! {
+    static ref ROOT_DIR: PathBuf = [env!("CARGO_MANIFEST_DIR"), ".."].iter().collect();
+    static ref FIXTURES_DIR: PathBuf = ROOT_DIR.join("test").join("fixtures");
+    static ref HEADER_DIR: PathBuf = ROOT_DIR.join("lib").join("include");
+    static ref GRAMMARS_DIR: PathBuf = ROOT_DIR.join("test").join("fixtures").join("grammars");
+    static ref SCRATCH_DIR: PathBuf = {
+        let result = ROOT_DIR.join("target").join("scratch");
+        fs::create_dir_all(&result).unwrap();
+        result
+    };
+    static ref TEST_LOADER: Loader = Loader::new(SCRATCH_DIR.clone());
+}
+
+pub fn fixtures_dir<'a>() -> &'static Path {
+    &FIXTURES_DIR
+}
+
+pub fn get_language(name: &str) -> Language {
+    TEST_LOADER
+        .load_language_at_path(name, &GRAMMARS_DIR.join(name).join("src"), &HEADER_DIR)
+        .unwrap()
+}
+
+pub fn get_test_language(name: &str, parser_code: String, path: &Path) -> Language {
+    let parser_c_path = SCRATCH_DIR.join(&format!("{}-parser.c", name));
+    if !fs::read_to_string(&parser_c_path)
+        .map(|content| content == parser_code)
+        .unwrap_or(false)
+    {
+        fs::write(&parser_c_path, parser_code).unwrap();
+    }
+    let scanner_path = path.join("scanner.c");
+    let scanner_path = if scanner_path.exists() {
+        Some(scanner_path)
+    } else {
+        None
+    };
+    TEST_LOADER
+        .load_language_from_sources(
+            name,
+            &HEADER_DIR,
+            &parser_c_path,
+            &scanner_path,
+        )
+        .unwrap()
+}