This patch intends to clean up some coding style issues in
VertexAttributeTest.cpp, which is helpful to implement Vertex
Attrib Binding.
1. Use std::string directly instead of macro SHADER_SOURCE
2. Use constexpr on constants
3. Use std::array instead of traditional C-style arrays

BUG=angleproject:1593

Change-Id: I5b6f6d4fe0e34e82a9fc38cb54932fbc39e19a80
Reviewed-on: https://chromium-review.googlesource.com/467406
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>