gh-149496: Fix MacOSTest.test_default regression when BROWSER env var is set (GH-149579)

gh-149496: Fix MacOSTest.test_default failing when BROWSER env var is set

MacOSTest.test_default calls webbrowser.get() and asserts it returns a
MacOS instance. When BROWSER is set in the environment (e.g. BROWSER=open,
a common macOS workaround for the old osascript-based implementation),
register_standard_browsers() registers a GenericBrowser as the preferred
browser instead, causing the assertion to fail.

This is a regression introduced in gh-137586, which added MacOSTest and
moved test_default into it from MacOSXOSAScriptTest. MacOSXOSAScriptTest
had an identical setUp() guard added in gh-131254 specifically to fix this
same failure. The guard was not carried over to MacOSTest.

Add setUp() to MacOSTest to unset BROWSER for the duration of each test,
restoring the isolation that was already established as the correct pattern
for macOS webbrowser tests.