Commit 4f7abea96e added a test that uses Conn.Raw, which was added in
Go >= 1.13. The go-sqlite3 project runs tests with Go >= 1.11. Remove
the test from sqlite3_test.go, so it only runs with the correct
versions of Go.
Instead of adding a new test, modify the existing test that already
uses Conn.Raw() to check the type of driverConn.
The package is useless without cgo, but Go 1.7 will happily build it
since doc.go is a pure-Go file, even if CGO_ENABLED=0, like when
cross-compiling.
This is particularly problematic since the package is usually imported
for its side effects, and users would successfully build a broken binary.
Workaround golang/go#16981
