Add Linux CI variant

Author: kateinoigakukun

.github/workflows/test.yml

@@ -5,10 +5,12 @@ jobs:
     name: Build and Test
     strategy:
       matrix:
-        os: [macOS-10.15]
+        os: [macOS-10.15, Ubuntu-18.04]
         include:
           - os: macOS-10.15
             toolchain: https://github.com/swiftwasm/swift/releases/download/swift-wasm-DEVELOPMENT-SNAPSHOT-2020-04-27-a/swift-wasm-DEVELOPMENT-SNAPSHOT-2020-04-27-a-osx.tar.gz
+          - os: Ubuntu-18.04
+            toolchain: https://github.com/swiftwasm/swift/releases/download/swift-wasm-DEVELOPMENT-SNAPSHOT-2020-04-27-a/swift-wasm-DEVELOPMENT-SNAPSHOT-2020-04-27-a-linux.tar.gz
     runs-on: ${{ matrix.os }}
     steps:
       - name: Checkout