[clang-tidy] Fix ClangTidyTest to initialize context before checks.

Summary:
Currently, `clang::tidy::test::runCheckOnCode()` constructs the check
instances *before* initializing the ClangTidyContext. This ordering causes
problems when the check's constructor accesses the context, for example, through
`getLangOpts()`.

This revision moves the construction to after the context initialization, which
follows the pattern used in the clang tidy tool itself.

Reviewers: gribozavr

Subscribers: xazax.hun, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63784

llvm-svn: 364435