Add small obs file with 5 obs/ 3 records for BC by record tests

[shlyaeva]

Description

Supports VarBC by record testing: small but meaningful new obs file. The file is generated from the existing testinput_tier_1/aircraft_obs_2018041500_s.nc4 by extracting 5 observations. The new file has a total of three records (2 obs, 1 obs, 2 obs). This is used to cover testing of MPI-distributed VarBC by record: for test on 2 MPI tasks the number of records on each task is different, which is an important test case to cover.

Impact

Supports https://github.com/JCSDA-internal/ufo/pull/3481

Checklist

• [x] I have performed a self-review of my own code
• [x] I have run the unit tests before creating the PR