Results of find_program('some-script") should be consistent, irrespective of value of PATHEXT

test_find_program() (a windows-only test) fails if .py isn't in PATHEXT

C:\meson>py run_unittests.py WindowsTests.test_find_program
F
Stdout:
Program cmd found: YES (C:\WINDOWS\system32\cmd.EXE)
Program cmd.exe found: YES (C:\WINDOWS\system32\cmd.exe)
Program C:\WINDOWS\system32\cmd found: YES (C:\WINDOWS\system32\cmd.exe)
Program C:\cygwin64\wip\meson\test cases/windows\8 find program\test-script found: YES (python C:\cygwin64\wip\meson\test cases/windows\8 find program\test-script)
Program C:\cygwin64\wip\meson\test cases/windows\8 find program\test-script-ext.py found: YES (C:\Users\jon\AppData\Local\Programs\Python\Python36\python.exe C:\cygwin64\wip\meson\test cases/windows\8 find program\test-script-ext.py)
Program test-script-ext found: NO

======================================================================
FAIL: test_find_program (__main__.WindowsTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "run_unittests.py", line 3041, in test_find_program
    self.assertTrue(prog.found(), msg='test-script-ext not found in PATH')
AssertionError: False is not true : test-script-ext not found in PATH

Stdout:
Program cmd found: YES (C:\WINDOWS\system32\cmd.EXE)
Program cmd.exe found: YES (C:\WINDOWS\system32\cmd.exe)
Program C:\WINDOWS\system32\cmd found: YES (C:\WINDOWS\system32\cmd.exe)
Program C:\cygwin64\wip\meson\test cases/windows\8 find program\test-script found: YES (python C:\cygwin64\wip\meson\test cases/windows\8 find program\test-script)
Program C:\cygwin64\wip\meson\test cases/windows\8 find program\test-script-ext.py found: YES (C:\Users\jon\AppData\Local\Programs\Python\Python36\python.exe C:\cygwin64\wip\meson\test cases/windows\8 find program\test-script-ext.py)
Program test-script-ext found: NO

----------------------------------------------------------------------
Ran 1 test in 0.059s

FAILED (failures=1)

C:\meson>echo %PATHEXT%
.COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.JSE;.WSF;.WSH;.MSC

C:\meson>set PATHEXT=%PATHEXT%;.py

C:\meson>py run_unittests.py WindowsTests.test_find_program
.
----------------------------------------------------------------------
Ran 1 test in 0.025s

OK

Start with run_unittests.py and WindowsTests.test_find_program, then reproduce the failure with and without .py in PATHEXT. Check the find_program behavior for test-script-ext and its explicit .py variant; done means the test passes consistently regardless of PATHEXT.