Improve unittest framework to have a virtual FS

[Geod24]

This gets the unittest framework closer to the actual behavior, and allows us to mock basic FS operations such as mkdir and writeFile. Further improvements can be made to support more operations, and to have package setup done before TestDub instantiation.

[github-actions]

✅ PR OK, no changes in deprecations or warnings

Total deprecations: 0

Total warnings: 0

Build statistics:

 statistics (-before, +after)
-executable size=5339480 bin/dub
-rough build time=62s
+executable size=5339488 bin/dub
+rough build time=61s

Full build output

DUB version 1.35.1, built on Jan  6 2024
LDC - the LLVM D compiler (1.36.0):
  based on DMD v2.106.1 and LLVM 17.0.6
  built with LDC - the LLVM D compiler (1.36.0)
  Default target: x86_64-unknown-linux-gnu
  Host CPU: znver3
  http://dlang.org - http://wiki.dlang.org/LDC


  Registered Targets:
    aarch64     - AArch64 (little endian)
    aarch64_32  - AArch64 (little endian ILP32)
    aarch64_be  - AArch64 (big endian)
    amdgcn      - AMD GCN GPUs
    arm         - ARM
    arm64       - ARM64 (little endian)
    arm64_32    - ARM64 (little endian ILP32)
    armeb       - ARM (big endian)
    avr         - Atmel AVR Microcontroller
    bpf         - BPF (host endian)
    bpfeb       - BPF (big endian)
    bpfel       - BPF (little endian)
    hexagon     - Hexagon
    lanai       - Lanai
    loongarch32 - 32-bit LoongArch
    loongarch64 - 64-bit LoongArch
    mips        - MIPS (32-bit big endian)
    mips64      - MIPS (64-bit big endian)
    mips64el    - MIPS (64-bit little endian)
    mipsel      - MIPS (32-bit little endian)
    msp430      - MSP430 [experimental]
    nvptx       - NVIDIA PTX 32-bit
    nvptx64     - NVIDIA PTX 64-bit
    ppc32       - PowerPC 32
    ppc32le     - PowerPC 32 LE
    ppc64       - PowerPC 64
    ppc64le     - PowerPC 64 LE
    r600        - AMD GPUs HD2XXX-HD6XXX
    riscv32     - 32-bit RISC-V
    riscv64     - 64-bit RISC-V
    sparc       - Sparc
    sparcel     - Sparc LE
    sparcv9     - Sparc V9
    spirv32     - SPIR-V 32-bit
    spirv64     - SPIR-V 64-bit
    systemz     - SystemZ
    thumb       - Thumb
    thumbeb     - Thumb (big endian)
    ve          - VE
    wasm32      - WebAssembly 32-bit
    wasm64      - WebAssembly 64-bit
    x86         - 32-bit X86: Pentium-Pro and above
    x86-64      - 64-bit X86: EM64T and AMD64
    xcore       - XCore
   Upgrading project in /home/runner/work/dub/dub/
    Starting Performing "release" build using /opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/ldc2 for x86_64.
    Building dub 1.36.0-beta.1+commit.63.gf61d23be: building configuration [application]
     Linking dub
STAT:statistics (-before, +after)
STAT:executable size=5339488 bin/dub
STAT:rough build time=61s

[Geod24]

There's a mismatch between what our vibecompat path offers and what vibe-core has. Fix in: #2821

[Geod24]

I used a different approach since #2821 introduces its own set of issues.