Use GitHub C syntax highlighting on test files
Use GitHub C syntax highlighting on test files

Add a .gitattributes file that tells GitHub to highlight all .function
files as if they were .c files. This aids in reviewing changes to tests.

Signed-off-by: David Horstmann <0386b5e8f8e5ea8a58d40026a5369df20e1c750d@arm.com>

ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls