unittests: allow test commands to suppress returning stderr

Add better logging of commands by printing stdout and stderr separately,
if relevant. Also while we are at it, print the actual command being
run.

unittests/baseplatformtests.py

@@ -32,7 +32,7 @@ import mesonbuild.environment
 import mesonbuild.coredata
 import mesonbuild.modules.gnome
 from mesonbuild.mesonlib import (
-    is_cygwin, windows_proof_rmtree, python_command
+    is_cygwin, join_args, windows_proof_rmtree, python_command
 )
 import mesonbuild.modules.pkgconfig
 
@@ -143,7 +143,7 @@ class BasePlatformTests(TestCase):
         os.environ.update(self.orig_env)
         super().tearDown()
 
-    def _run(self, command, *, workdir=None, override_envvars: T.Optional[T.Mapping[str, str]] = None):
+    def _run(self, command, *, workdir=None, override_envvars: T.Optional[T.Mapping[str, str]] = None, stderr=True):
         '''
         Run a command while printing the stdout and stderr to stdout,
         and also return a copy of it
@@ -158,10 +158,16 @@ class BasePlatformTests(TestCase):
             env.update(override_envvars)
 
         p = subprocess.run(command, stdout=subprocess.PIPE,
-                           stderr=subprocess.STDOUT, env=env,
+                           stderr=subprocess.STDOUT if stderr else subprocess.PIPE,
+                           env=env,
                            encoding='utf-8',
                            text=True, cwd=workdir, timeout=60 * 5)
+        print('$', join_args(command))
+        print('stdout:')
         print(p.stdout)
+        if not stderr:
+            print('stderr:')
+            print(p.stderr)
         if p.returncode != 0:
             if 'MESON_SKIP_TEST' in p.stdout:
                 raise SkipTest('Project requested skipping.')
@@ -235,13 +241,13 @@ class BasePlatformTests(TestCase):
                 out = self._get_meson_log()  # best we can do here
         return out
 
-    def build(self, target=None, *, extra_args=None, override_envvars=None):
+    def build(self, target=None, *, extra_args=None, override_envvars=None, stderr=True):
         if extra_args is None:
             extra_args = []
         # Add arguments for building the target (if specified),
         # and using the build dir (if required, with VS)
         args = get_builddir_target_args(self.backend, self.builddir, target)
-        return self._run(self.build_command + args + extra_args, workdir=self.builddir, override_envvars=override_envvars)
+        return self._run(self.build_command + args + extra_args, workdir=self.builddir, override_envvars=override_envvars, stderr=stderr)
 
     def clean(self, *, override_envvars=None):
         dir_args = get_builddir_target_args(self.backend, self.builddir, None)