testscript: support Go 1.16, move to Actions

https://go-review.googlesource.com/c/go/+/250977 adds a new method to
the testing.testDeps interface, so mirror that here in our no-op
implementation too.

While at it, replace Travis with GitHub Actions, which is faster and
easier to use. We still test on Linux, Mac, and Windows, and with the
two latest Go versions. Otherwise, the config is a straight port, and we
run the same commands.

.github/workflows/test.yml

@@ -0,0 +1,28 @@
+on: [push, pull_request]
+name: Test
+jobs:
+  test:
+    strategy:
+      matrix:
+        go-version: [1.14.x, 1.15.x]
+        os: [ubuntu-latest, macos-latest, windows-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+    - name: Install Go
+      uses: actions/setup-go@v2
+      with:
+        go-version: ${{ matrix.go-version }}
+    - name: Checkout code
+      uses: actions/checkout@v2
+    - name: Test
+      run: |
+        go test ./...
+        go test -race ./...
+
+    - name: Tidy
+      if: matrix.os == 'ubuntu-latest' # no need to do this everywhere
+      run: |
+        go mod tidy
+
+        test -z "$(gofmt -d .)" || (gofmt -d . && false)
+        test -z "$(git status --porcelain)" || (git status; git diff && false)

testscript/exe.go

@@ -211,3 +211,6 @@ func (nopTestDeps) WriteHeapProfile(io.Writer) error {
 	// Not needed for Go 1.10.
 	return nil
 }
+
+// Note: SetPanicOnExit0 was added in Go 1.16.
+func (nopTestDeps) SetPanicOnExit0(bool) {}

testscript/testdata/wait.txt

@@ -1,6 +1,10 @@
 [!exec:echo] skip
 [!exec:false] skip
 
+# TODO: the '\n' below doesn't work on Windows on Github Actions, which does
+# have coreutils like "echo" installed. Perhaps they emit CRLF?
+[windows] skip
+
 exec echo foo
 stdout foo