Disable the use of ANSI sequences in logging when --nocolor is passed

--nocolor is an existing option defined in e3.main. e3-testsuite has
heuristics in order to detect if it is safe to use ANSI sequences in
logging in order to colorize the output in the console, but --nocolor
should have precedence over these heuristics.

No testcase added as tests run without TTYs, and so colors are always
disabled there.

For #113

NEWS.md

@@ -1,6 +1,7 @@
 26.0 (Not released yet)
 =======================
 
+* Disable the use of ANSI sequences in logging when `--nocolor` is passed.
 * Make `DiffTestDriver` perform a full match for regexp baselines.
 * Fix memory leaks for test driver data.
 * Include total testsuite and testcase durations in XUnit reports.

src/e3/testsuite/__init__.py

@@ -414,6 +414,11 @@ def testsuite_main(self, args: Optional[List[str]] = None) -> int:
         else:  # interactive-only
             enable_colors = True
             self.main.args.show_error_output = True
+
+        # If explicitly requested, disable colors
+        if self.main.args.nocolor:
+            enable_colors = False
+
         self.colors = ColorConfig(enable_colors)
         self.Fore = self.colors.Fore
         self.Style = self.colors.Style