Add environment variable knobs to customise the E2E test runs.

[otrho]

The harness will now check for the existence (but not care about the
value) of two environment variables while running tests:

• SWAY_TEST_VERBOSE - this disables silent_mode and prints warnings
  and errors to stdout.
• SWAY_TEST_USE_IR - this turns on the use_ir flag to run the tests
  using the IR pipeline (useful presently until IR becomes the default).

E.g.,

$ cd sway/test
$ SWAY_TEST_VERBOSE=yes cargo r neq
    Finished dev [unoptimized + debuginfo] target(s) in 0.17s
     Running `sway/target/debug/test neq`
 Compiling neq_4_test
  Compiled library "core".
  Compiled library "core".
warning
 --> FuelLabs-sway-lib-std-5a0938f/src/result.sw:0:5
  |
...
3 |     Ok: T,
  |     -- Enum variant Ok is never constructed.
  |
warning
 --> FuelLabs-sway-lib-std-5a0938f/src/result.sw:0:5
  |
...
4 |     Err: E,
  |     --- Enum variant Err is never constructed.
  |
  Compiled library "lib-std" with 2 warnings.
  Compiled script "neq_4_test".
  Bytecode size is 108 bytes.
   ABI gen neq_4_test
  Compiled library "core".
  Compiled library "core".
  Compiled library "lib-std" with 2 warnings.
  Compiled script "neq_4_test".
_________________________________
Tests passed.
1 tests run (81 skipped)

[adlerjohn]

Is there a reason to use environment variables instead of CLI options?

[otrho]

Is there a reason to use environment variables instead of CLI options?

Only that it's simpler. Don't have to parse them and the pass them all the way from main(). I prefer it but maybe that's subjective.

[sezna]

I like the env vars.