base: Rename TestFiber into SwitchingFiber
base: Rename TestFiber into SwitchingFiber

This is done since TestFiber is too generic and it is not the only
Fiber's testing subclass in the unit test

Change-Id: Idc386f487091ae9bdadae865090a6719a25583da
Signed-off-by: Giacomo Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/19153
Reviewed-by: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Maintainer: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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