src/sage/env.py: canonicalize paths in a test

A test in sage.env is running sage in a subprocess to compare the
values of SAGE_ROOT and SAGE_LOCAL. It does the comparison as strings,
however, and can fail:

  File "src/sage/env.py", line 14, in sage.env
  Failed example:
      out == repr((SAGE_ROOT, SAGE_LOCAL))   # long time
  Expected:
      True
  Got:
      False

This despite the fact that both values are equivalent:

  sage: out
  "('/home/mjo/src/sage.git/src/sage/../..', '/usr')"
  sage: repr((SAGE_ROOT, SAGE_LOCAL))
  "('/home/mjo/src/sage.git', '/usr')"

We update the test to canonicalize the paths within the subprocess,
and output only "True" or "False" instead.

src/sage/env.py

@@ -8,10 +8,14 @@
 
     sage: env = {k:v for (k,v) in os.environ.items() if not k.startswith("SAGE_")}
     sage: from subprocess import check_output
-    sage: environment = "sage.all"
-    sage: cmd = f"from {environment} import SAGE_ROOT, SAGE_LOCAL; print((SAGE_ROOT, SAGE_LOCAL))"
+    sage: module_name = "sage.all"   # hide .all import from the linter
+    sage: cmd  =  "from {module_name} import SAGE_ROOT, SAGE_LOCAL;"
+    sage: cmd +=  "from os.path import samefile;"
+    sage: cmd += f"s1 = samefile(SAGE_ROOT, '{SAGE_ROOT}');"
+    sage: cmd += f"s2 = samefile(SAGE_LOCAL, '{SAGE_LOCAL}');"
+    sage: cmd += "print(s1 and s2);"
     sage: out = check_output([sys.executable, "-c", cmd], env=env).decode().strip()   # long time
-    sage: out == repr((SAGE_ROOT, SAGE_LOCAL))                                        # long time
+    sage: out == "True"                                                               # long time
     True
 
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