fix: Fixing the custom test strategy override from vim-test

Using the correct variable name (from vim-test) so that any original custom strategies are preserved and extended by ultest

Rationale: I have been using custom strategies in vim-test, and when I installed ultest (which is fantastic by the way!) I noticed my custom strategies were no longer available. I realised this is because the custom strategies variable that is retrieved in the ultest code (presumably with the intention of adding on the ultest#handler#strategy) is using a different variable name to the one use by vim-test (see https://github.com/vim-test/vim-test#custom-strategies), so it blows away any custom strategies that had been set using this original vim-test global variable.

I worked around this by updating my config to set test#custom#strategies instead of test#custom_strategies, but believe it should be fixed so that the variable names are consistent between the two plugins.

Of course, anyone else who happened to use this workaround as I did, will then have to revert back if this fix goes in.

rcarriga / vim-ultest

fix: Fixing the custom test strategy override from vim-test #62