Modify test script to accept shells as arguments
Modify test script to accept shells as arguments

While not useful in CI scripts, this lets a developer run the tests
against their own shells if they wish.

I tested this with:
bash 3.2.0(1)-release (x86_64-unknown-linux-gnu)
bash 4.0.28(2)-release (x86_64-unknown-linux-gnu)
bash 4.2.53(1)-release (x86_64-unknown-linux-gnu)
bash 4.4.0(1)-release (x86_64-unknown-linux-gnu)
bash 4.4.19(1)-release (x86_64-unknown-linux-gnu)
bash 5.0.17(1)-release (x86_64-pc-linux-gnu)
bash 5.1.0(1)-release (x86_64-pc-linux-gnu)
zsh 5.0.8 (x86_64-unknown-linux-gnu)
zsh 5.1.1 (x86_64-unknown-linux-gnu)
zsh 5.2 (x86_64-unknown-linux-gnu)
zsh 5.3.1 (x86_64-unknown-linux-gnu)
zsh 5.4.2 (x86_64-unknown-linux-gnu)
zsh 5.5.1 (x86_64-redhat-linux-gnu)
zsh 5.8 (x86_64-pc-linux-gnu)

I found a few bugs, fixed in the following commits.

pkkolos/liquidprompt,laurentbardin/liquidprompt,nojhan/liquidprompt,milouse/liquidprompt,jaesivsm/liquidprompt