Run compiletest tests on CI

I was assuming that https://github.com/rust-lang/rust/pull/56792 would
have resulted in compiletest tests being executed in CI. However, I
couldn't find any mentions of the unit test names in any CI logs.

This adds the compiletest test execution to the checktools.sh script.

src/ci/docker/x86_64-gnu-tools/checktools.sh

@@ -37,6 +37,11 @@ python2.7 "$X_PY" test --no-fail-fast \
     src/tools/rls \
     src/tools/rustfmt \
     src/tools/miri \
+    src/tools/compiletest
+
+# Note that compiletest here is an exception from the other tools.
+# We are only executing the unit tests of it. We don't need to track the
+# toolstate because it's not distributed as a tool like the other items.
 
 set -e