Add automated tests

[sohang3112]

Test Setup:

import forth_kernel
kernel = forth_kernel.ForthKernel()

• Test that kernel.language_version is of the correct format (eg. Gforth 0.7.3)
• Test that Forth input code is being syntax highlighted properly (not sure how to test this)
• Test shell command: !echo hello world should return hello world in stdout. NOTE: This is currently failing - only single-word commands like ls are working.
• Do multiple Jupyter Notebooks continue working concurrently even if one hangs - eg. if I run long-running code in 1 Jupyter Notebook, and simultaneously run short code 1 2 + . in another Jupyter Notebook, does the second notebook still respond or does it hang until first notebook is done executing?
• Test that for this input:

1 2 + .
.
bye

Stdout output is (NOTE that the stdout output from kernel is in HTML, so first convert to text and then compare, ignoring whitespace):

1 2 + . 3  ok
. 
bye

Stderr error output is (strip whitespace; just check "Stack underflow", ignore text before that in first line:

:2: Stack underflow
>>>.<<<
Backtrace:
...
Killing kernel because GForth process has died