gas-report: make it work within the context of MudTest

[holic]

if you run mud-gas-report on a thing that extends MudTest, it'll fail because it's missing a .mudtest file (created on MUD deploy)

since our deploy pipeline is separate (a TS script) from the rest of the foundry lifecycle, I think we should:

• make gas-report (or sub command of) more of a pure "test output parser" rather than a "test runner"
• allow overriding the test command used in the gas-report test runner

another wild idea: get MudTest to call out to TS deploy script during the test run, but not sure if there's an exposed RPC within the test runner we can thread through to the deploy process via CLI arg