(Naming) convention for test files/directory

[neatudarius]

As per my check in beman/exemplar (Apply the Beman Standard to beman.exampler · Issue #14 · beman-project/exemplar · GitHub), the last 2 missing obvious parts from the Beman standard is the location and the naming for test files.

Proposals:

• Add TEST.LOCATION to the standard
  v1: have a tests subdirectory - e.g. optional26/src/beman/optional26/tests at main · beman-project/optional26 · GitHub
  v2: have test files near the code - e.g. exemplar/src/beman/exemplar at main · beman-project/exemplar · GitHub

• Add TEST.FILE_NAME
  v1: use *.t.cpp suffix. This can be extended to *.fail.t.cpp or *.pass.t.cpp or *.fail_compile.t.cpp - e.g. optional26/src/beman/optional26/tests at main · beman-project/optional26 · GitHub
  v2: use other suffix (e.g. *.pass.cpp) - execution26/src/beman/execution26/tests at main · beman-project/execution26 · GitHub

PS This is not a thread where we discuss the testing framework. Please check Test Infrastructure: preferably not Google Test! - #8 by jwakely.