* All: Drop 'basic-blocks' feature
This makes it so that 'basic-blocks' cannot be disabled and we can
start assuming it everywhere.
* Tests: Replace non-bb filetests with bb version
* Tests: Adapt solver-fixedconflict filetests to use basic blocks
Cranelift can be compiled with feature flags which can change its output. To
accomodate changes of output related to feature flags, test file can now include
`feature "..."` and `feature ! "..."` directives in the preamble of the test
file.
The test runner would skip the test if the flag does not match the expectation
of the test case.
* Add options for parsing test files
This change allows adding parsing parameters more easily; e.g. a parameter is needed for setting the default calling convention for functions parsed as a part of the `run` test feature.
* Set default calling convention that of the host for `test run` file tests
Previously `test run` used the parser's hard-coded CallConv::Fast as the default calling convention but with this change any test being `run` will use the default calling convention of the machine running the test. `test run` will now throw an error if the calling convention of the function does not match the host's.
