Add environment variable check to force console output

[timwoj]

Taken from @awelzel's suggestion in #143 (comment).

Fixes #143

[bbannier]

WDYT about adding a proper option for this, and also reading a default from the environment like we do for --progress-bar?

btest/btest

Lines 3026 to 3033 in 1e97bbe

	optparser.add_option(
	"-P",
	"--progress-bar",
	action="store_true",
	dest="osc94_progress",
	default=os.environ.get("BTEST_OSC94_PROGRESS", ""),
	help="display progress bar using OSC 9;4 escape sequences (set BTEST_OSC94_PROGRESS to permanently enable)",
	)

It would also be nice to add a test for this.