test: Test against file system

[AndrewSisley]

Closes #127

Todo:

• Set environment variable DEFRA_BADGER_FILE to non-default value for CI tests

[AndrewSisley]

Spoke over discord some things were agreed:

1. file based tests to be off by default (not just for performance, but not all dev environments can access file system in suitable manner)
2. have a look at using the test.tempDir functionality and maybe randomly generate sub-dir names (to remove/reduce need to run in serial
3. have a look at using go.test startup/teardown functions for cleanup (might not be suitable if each test has to actively do stuff for it)

[codecov]

Codecov Report

Merging #128 (0a6528d) into develop (746aec2) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #128   +/-   ##
========================================
  Coverage    54.09%   54.09%           
========================================
  Files           35       35           
  Lines         3629     3629           
========================================
  Hits          1963     1963           
  Misses        1437     1437           
  Partials       229      229           

Impacted Files	Coverage Δ
query/graphql/schema/relations.go	54.63% <0.00%> (ø)

[AndrewSisley]

Was way easier to do this than I expected - t.TempDir is super handy to have (it also handles teardown and provides a unique dir per test) :)

[jsimnz]

ezpz. LGTM