[tests] Don't make the mscorlib tests exit upon suspend. (#6115)

It means that on watchOS they'll exit if the screen turns off.

I have no idea why we set it up like this in the first place, it's been like
this since we created the mscorlib tests ([1]).

[1]: 76e15036e8

tests/bcl-test/mscorlib/Info.plist

@@ -6,8 +6,6 @@
 	<string>com.xamarin.mscorlibtests</string>
 	<key>MinimumOSVersion</key>
 	<string>6.0</string>
-	<key>UIApplicationExitsOnSuspend</key>
-	<true/>
 	<key>UIDeviceFamily</key>
 	<array>
 		<integer>1</integer>