py3: quote several instances of $PYTHON for Windows

Python3 lives in Program Files by default.  The last remaining unquoted instance
is in test-hghave.t.  I can't figure out how to quote that, as it either
complains:

    'c:\\Program' is not recognized as an internal or external command,
    operable program or batch file.

or

    $TESTTMP.sh: line 22: "C:/Program Files/Python37/python.exe": $ENOENT$

tests/test-extension.t

@@ -165,7 +165,7 b' Check that extensions are loaded in phas'
   > def custompredicate(repo, subset, x):
   >     return smartset.baseset([r for r in subset if r in {0}])
   > EOF
-  $ $PYTHON $TESTTMP/unflush.py foo.py
+  $ "$PYTHON" $TESTTMP/unflush.py foo.py
 
   $ cp foo.py bar.py
   $ echo 'foo = foo.py' >> $HGRCPATH
@@ -264,7 +264,7 b' limit mark, regardless of importing modu'
   > def extsetup():
   >     print('ambigabs.s=%s' % ambigabs.s, flush=True)
   > NO_CHECK_EOF
-  $ $PYTHON $TESTTMP/unflush.py loadabs.py
+  $ "$PYTHON" $TESTTMP/unflush.py loadabs.py
   $ (PYTHONPATH=${PYTHONPATH}${PATHSEP}${TESTTMP}/libroot; hg --config extensions.loadabs=loadabs.py root)
   ambigabs.s=libroot/ambig.py
   $TESTTMP/a
@@ -280,7 +280,7 b' limit mark, regardless of importing modu'
   > def extsetup():
   >     print('ambigrel.s=%s' % ambigrel.s, flush=True)
   > NO_CHECK_EOF
-  $ $PYTHON $TESTTMP/unflush.py loadrel.py
+  $ "$PYTHON" $TESTTMP/unflush.py loadrel.py
   $ (PYTHONPATH=${PYTHONPATH}${PATHSEP}${TESTTMP}/libroot; hg --config extensions.loadrel=loadrel.py root)
   ambigrel.s=libroot/mod/ambig.py
   $TESTTMP/a
@@ -1825,7 +1825,7 b' Prohibit the use of unicode strings as t'
   > def ext(*args, **opts):
   >     print(opts[b'opt'], flush=True)
   > EOF
-  $ $PYTHON $TESTTMP/unflush.py $TESTTMP/test_unicode_default_value.py
+  $ "$PYTHON" $TESTTMP/unflush.py $TESTTMP/test_unicode_default_value.py
   $ cat > $TESTTMP/opt-unicode-default/.hg/hgrc << EOF
   > [extensions]
   > test_unicode_default_value = $TESTTMP/test_unicode_default_value.py

tests/test-filebranch.t

@@ -6,7 +6,7 b' when we do a merge.'
   > import sys, os
   > print("merging for", os.path.basename(sys.argv[1]))
   > EOF
-  $ HGMERGE="$PYTHON ../merge"; export HGMERGE
+  $ HGMERGE="\"$PYTHON\" ../merge"; export HGMERGE
 
 Creating base:
 

tests/test-import.t

@@ -66,7 +66,7 b' regardless of the commit message in the '
   new changesets 80971e65b431
   updating to branch default
   2 files updated, 0 files merged, 0 files removed, 0 files unresolved
-  $ HGEDITOR=cat hg --config ui.patch="$PYTHON ../dummypatch.py" --cwd b import --edit ../exported-tip.patch
+  $ HGEDITOR=cat hg --config ui.patch="\"$PYTHON\" ../dummypatch.py" --cwd b import --edit ../exported-tip.patch
   applying ../exported-tip.patch
   second change
   

tests/test-merge1.t

@@ -11,7 +11,7 b''
   > 
   > print("merging for", os.path.basename(sys.argv[1]))
   > EOF
-  $ HGMERGE="$PYTHON ../merge"; export HGMERGE
+  $ HGMERGE="\"$PYTHON\" ../merge"; export HGMERGE
 
   $ hg init t
   $ cd t