goose: read tests from testdata

Since tests are all written in terms of Go source, it makes tests easier
to write.  They still need to be arranged in a GOPATH for go build, but
tests that just call Generate can operate in-place because I've faked
the filesystem.

Reviewed-by: Tuo Shan <shantuo@google.com>
Reviewed-by: Herbie Ong <herbie@google.com>

internal/goose/testdata/InjectInputConflict/foo/foo.go

@@ -0,0 +1,23 @@
+package main
+
+import "fmt"
+
+func main() {
+	// I'm on the fence as to whether this should be an error (versus an
+	// override). For now, I will make it an error that can be relaxed
+	// later.
+	fmt.Println(injectBar(40))
+}
+
+type Foo int
+type Bar int
+
+//goose:provide Set
+func provideFoo() Foo {
+	return -888
+}
+
+//goose:provide Set
+func provideBar(foo Foo) Bar {
+	return 2
+}

internal/goose/testdata/InjectInputConflict/foo/foo_goose.go

@@ -0,0 +1,7 @@
+//+build gooseinject
+
+package main
+
+//goose:use Set
+
+func injectBar(foo Foo) Bar