C++: Turn header variant tests that use PCH files into integration tests

These tests currently test a code path in the extractor that only exists for
these tests. By turning them into integration tests, we actually use the
code path that normal database creation uses.

Author: jketema

cpp/ql/integration-tests/header-variant-tests/clang-pch/a.c

@@ -0,0 +1,2 @@
+#include "a.h"
+#define FOUR 4

cpp/ql/test/header-variant-tests/clang-pch/a.h renamed to cpp/ql/integration-tests/header-variant-tests/clang-pch/a.h

@@ -0,0 +1,3 @@
+int main() {
+  return ONE + FOUR;
+}

cpp/ql/test/header-variant-tests/clang-pch/b.c renamed to cpp/ql/integration-tests/header-variant-tests/clang-pch/b.c

@@ -0,0 +1 @@
+#import "d.h"

cpp/ql/test/header-variant-tests/clang-pch/b.h renamed to cpp/ql/integration-tests/header-variant-tests/clang-pch/b.h

@@ -0,0 +1,3 @@
+int main() {
+  return SEVENTEEN;
+}