nixos/tests/non-default-filesystems: fix btrfs mount regex

[ghpzin]

• fix regex checking mount to handle other mount options
• add output from /proc/mounts with '/dev/vda' for debug purposes

Description of changes

Fixes nixosTests.non-default-filesystems.btrfs (fails since 2024-02-29):
https://hydra.nixos.org/job/nixos/trunk-combined/nixos.tests.non-default-filesystems.btrfs.x86_64-linux
https://hydra.nixos.org/build/271668780
Error log:

(finished: waiting for unit multi-user.target, in 17.47 seconds)
subtest: BTRFS filesystems are mounted correctly
machine: must succeed: grep -E '/dev/vda / btrfs rw,relatime,space_cache=v2,subvolid=[0-9]+,subvol=/root 0 0' /proc/mounts
machine: output: 
Test "BTRFS filesystems are mounted correctly" failed with error: "command `grep -E '/dev/vda / btrfs rw,relatime,space_cache=v2,subvolid=[0-9]+,subvol=/root 0 0' /proc/mounts` failed (exit code 1)"

(there is additional discard=async default mount option now, so previous regex returns nothing):

/dev/vda / btrfs rw,relatime,discard=async,space_cache=v2,subvolid=256,subvol=/root 0 0
/dev/vda /home btrfs rw,relatime,discard=async,space_cache=v2,subvolid=257,subvol=/home 0 0

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[ghpzin]

@ofborg test non-default-filesystems.btrfs