Fix unit test Stage mocking

[teclator]

Fix for failing build:

[  122s] 
[  122s]   1) Y2Autoinstallation::EFIDetector.boot_efi? when called in normal Mode returns true if '/sys/firmware/efi/efivars' exists
[  122s]      Failure/Error: expect(described_class.boot_efi?).to eq(true)
[  122s] 
[  122s]        expected: true
[  122s]             got: false
[  122s] 
[  122s]        (compared using ==)
[  122s] 
[  122s]        Diff:
[  122s]        @@ -1,2 +1,2 @@
[  122s]        -true
[  122s]        +false
[  122s]      # ./test/lib/efi_detector_test.rb:36:in `block (5 levels) in <top (required)>'
[  122s] 
[  122s]   2) Y2Autoinstallation::EFIDetector.boot_efi? when called in normal Mode returns true if '/sys/firmware/efi/vars/' exists
[  122s]      Failure/Error: expect(described_class.boot_efi?).to eq(true)
[  122s] 
[  122s]        expected: true
[  122s]             got: false
[  122s] 
[  122s]        (compared using ==)
[  122s] 
[  122s]        Diff:
[  122s]        @@ -1,2 +1,2 @@
[  122s]        -true
[  122s]        +false
[  122s]      # ./test/lib/efi_detector_test.rb:36:in `block (5 levels) in <top (required)>'

https://build.suse.de/package/live_build_log/Devel:YaST:Head/autoyast2/SUSE_SLE-15-SP4_GA/x86_64

[coveralls]

Coverage remained the same at 68.048% when pulling 97188f4 on fix_unit_test_mocking into 8846b15 on master.

[yast-bot]

✔️ Public Jenkins job #216 successfully finished

[yast-bot]

✔️ Internal Jenkins job #128 successfully finished
✔️ Created IBS submit request #259707