Colorizing test output

[hugoleborso]

made with @EwenQuim

Summary

We added colorizing to the terminal output of tests, while avoiding to print escaping characters when the output is outside a classic terminal.

Changes

more changes to come, for now only the Equal function has been colorized

• Added one non exported global variable to know whether the program is ran in terminal
• Added a colors.go file handling colorization in the assert package

Motivation

Users have been requesting to color the output of tests (#946)
Lots of PR to colorize the result do not take into account whether the output is in a terminal or not

Example usage

In terminal	In a file

Performance

Inside a terminal - small performance loss

Benchmark	iterations	time/op	mem size alloc/op	alloc/op
BenchmarkColored/benchMarkingString-10	24840926	47.39 ns/op	24 B/op	1 allocs/op
BenchmarkColored/benchMarkingStruct-10	8493196	140.9 ns/op	72 B/op	2 allocs/op
BenchmarkEqual/Base-10	5313129	225.9 ns/op	112 B/op	3 allocs/op
BenchmarkEqual/Colored_(test_with_and_without_terminal)-10	2945031	409.1 ns/op	288 B/op	7 allocs/op

Outside a terminal - no performance loss

Benchmark	iterations	time/op	mem size alloc/op	alloc/op
BenchmarkColored/benchMarkingString-10	1000000000	0.3080 ns/op	0 B/op	0 allocs/op
BenchmarkColored/benchMarkingStruct-10	59117062	20.46 ns/op	24 B/op	1 allocs/op
BenchmarkEqual/Base-10	5326148	225.5 ns/op	112 B/op	3 allocs/op
BenchmarkEqual/Colored_(test_with_and_without_terminal)-10	4611681	259.6 ns/op	128 B/op	3 allocs/op

Next steps

We only colorized the Equal function, but if this pleases the maintainers we can also colorized all calls to Fail, and are open to suggestions

Related issues

Closes #946