scripts/ tests/: avoid spurious test failure with sysinstall.

scripts/sysinstall.py:
    Set PYMUPDF_SYSINSTALL_TEST=1 when running tests.

tests/test_pixmap.py:test_3854():
    Accept non-zero rms if PYMUPDF_SYSINSTALL_TEST is 1.

tests/test_pylint.py:
    Allow `if 0 and ...`:

scripts/sysinstall.py

@@ -190,8 +190,8 @@ def main():
     sudo = ''
     if root == '/':
         sudo = f'sudo PATH={os.environ["PATH"]} '
-    def run(command):
-        return run_command(command, doit=mupdf_do)
+    def run(command, env_extra=None):
+        return run_command(command, doit=mupdf_do, env_extra=env_extra)
     # Get MuPDF from git if specified.
     #
     if mupdf_git:
@@ -334,8 +334,8 @@ def run(command):
     # Run pytest tests.
     #
     log('## Run PyMuPDF pytest tests.')
-    def run(command):
-        return run_command(command, doit=pytest_do)
+    def run(command, env_extra=None):
+        return run_command(command, doit=pytest_do, env_extra=env_extra)
     import gh_release
     if pip == 'venv':
         # Create venv.
@@ -387,12 +387,12 @@ def run(command):
         command += f' -p {shlex.quote(pytest_args)}'
     if pytest_do:
         command += ' test'
-    run(command)
+    run(command, env_extra=dict(PYMUPDF_SYSINSTALL_TEST='1'))
 
 
-def run_command(command, capture=False, check=True, doit=True):
+def run_command(command, capture=False, check=True, doit=True, env_extra=None):
     if doit:
-        return pipcl.run(command, capture=capture, check=check, caller=2)
+        return pipcl.run(command, capture=capture, check=check, caller=2, env_extra=env_extra)
     else:
         log(f'## Would have run: {command}', caller=2)
 

tests/test_pixmap.py

@@ -426,4 +426,9 @@ def test_3854():
 
     rms = gentle_compare.pixmaps_rms(pixmap, pixmap_expected)
     print(f'{rms=}.')
-    assert rms == 0
+    if os.environ.get('PYMUPDF_SYSINSTALL_TEST') == '1':
+        # MuPDF using external third-party libs gives slightly different
+        # behaviour.
+        assert rms < 1
+    else:
+        assert rms == 0

tests/test_pylint.py

@@ -36,6 +36,7 @@ def test_pylint():
             W0622: Redefining built-in 'open' (redefined-builtin)
             W1309: Using an f-string that does not have any interpolated variables (f-string-without-interpolation)
             R1734: Consider using [] instead of list() (use-list-literal)
+            R1727: Boolean condition '0 and g_exceptions_verbose' will always evaluate to '0' (condition-evals-to-constant)
             '''
             )