Tests: Mock IO in the `Dub` class too

[Geod24]

This will allow to write more tests for the Dub class. In particular I am interested in being able to write unittests around the config file, which require loadConfig to be callable. We still need to call thisExePath at the moment but that can be solved later as it has no visible side effect.

[github-actions[bot]]

✅ PR OK, no changes in deprecations or warnings

Total deprecations: 8

Total warnings: 0

Build statistics:

 statistics (-before, +after)
-executable size=5297464 bin/dub
+executable size=5301560 bin/dub
 rough build time=63s

Full build output

``` DUB version 1.38.0, built on Jul 4 2024 LDC - the LLVM D compiler (1.39.0): based on DMD v2.109.1 and LLVM 18.1.6 built with LDC - the LLVM D compiler (1.39.0) 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 spirv - SPIR-V Logical 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/ldc2-1.39.0/x64/ldc2-1.39.0-linux-x86_64/bin/ldc2 for x86_64. Building dub 1.39.0-beta.1+commit.27.gaa1cf422: building configuration [application] source/dub/internal/dyaml/composer.d(210,43): Deprecation: cannot access overlapped field `Event.implicit` with unsafe bit patterns in `@safe` code source/dub/internal/dyaml/composer.d(232,43): Deprecation: cannot access overlapped field `Event.implicit` with unsafe bit patterns in `@safe` code source/dub/internal/dyaml/composer.d(336,43): Deprecation: cannot access overlapped field `Event.implicit` with unsafe bit patterns in `@safe` code source/dub/internal/dyaml/event.d(196,5): Deprecation: cannot access overlapped field `Event.explicitDocument` with unsafe bit patterns in `@safe` code source/dub/internal/dyaml/event.d(214,5): Deprecation: cannot access overlapped field `Event.explicitDocument` with unsafe bit patterns in `@safe` code source/dub/internal/dyaml/event.d(241,5): Deprecation: cannot access overlapped field `Event.implicit` with unsafe bit patterns in `@safe` code source/dub/internal/dyaml/event.d(148,5): Deprecation: cannot access overlapped field `Event.implicit` with unsafe bit patterns in `@safe` code source/dub/internal/dyaml/event.d(148,5): Deprecation: cannot access overlapped field `Event.implicit` with unsafe bit patterns in `@safe` code Linking dub STAT:statistics (-before, +after) STAT:executable size=5301560 bin/dub STAT:rough build time=63s ```