Export main and execute _initialize and main in test entry

IntegrationTests are unintentionally disabled since
0a38d70 because reactor mode doesn't
export `_start` and `WASI.start` doesn't do anything except setting the
instance to WASI module. So export `main` manually, and call
`_initialize` according to the "new-style commands" described in
https://github.com/WebAssembly/WASI/blob/59cbe140561db52fc505555e859de884e0ee7f00/legacy/application-abi.md#current-unstable-abi

Symbol support (081784b) broke tests
silently, so revert it for now.

Author: kateinoigakukun

IntegrationTests/Makefile

@@ -8,6 +8,7 @@ TestSuites/.build/$(CONFIGURATION)/%.wasm: FORCE
 	            --triple wasm32-unknown-wasi \
 	            --configuration $(CONFIGURATION) \
 	            -Xswiftc -Xclang-linker -Xswiftc -mexec-model=reactor \
+		    -Xlinker --export=main \
 		    $(SWIFT_BUILD_FLAGS)
 
 dist/%.wasm: TestSuites/.build/$(CONFIGURATION)/%.wasm

IntegrationTests/lib.js

@@ -45,6 +45,8 @@ const startWasiTask = async (wasmPath) => {
     swift.setInstance(instance);
     // Start the WebAssembly WASI instance!
     wasi.start(instance);
+    instance.exports._initialize();
+    instance.exports.main();
 };
 
 module.exports = { startWasiTask };