Set LLVM_INCLUDE_TESTS=OFF when building LLVM

When LLVM_INCLUDE_TESTS is enabled (by default), LLVM requires
Python 3.6 for the lit test runner, otherwise only Python 3.0 is
required.

As we have many docker images using Ubuntu 16.04, which only has
Python 3.5, this avoids the need to install a newer Python version
for them.

src/bootstrap/native.rs

@@ -180,6 +180,7 @@ impl Step for Llvm {
             .define("LLVM_INCLUDE_EXAMPLES", "OFF")
             .define("LLVM_INCLUDE_DOCS", "OFF")
             .define("LLVM_INCLUDE_BENCHMARKS", "OFF")
+            .define("LLVM_INCLUDE_TESTS", "OFF")
             .define("LLVM_ENABLE_TERMINFO", "OFF")
             .define("LLVM_ENABLE_LIBEDIT", "OFF")
             .define("LLVM_ENABLE_BINDINGS", "OFF")