Use resource limits instead of setting process directives in nf-test configuration

[adamrtalbot]

Description of feature

Similar to #3037, we should set the maximum cpu, memory and time limit on nf-tests. This can be mostly done by the -profile test however it is beneficial to set a hard limit when using nf-tests so accidentally not using the test profile does not cause issues.

See https://github.com/nf-core/rnaseq/blob/99fb7efc9d2b17a7940f745570ff162c04548bc4/tests/nextflow.config#L23-L30 for current implementation in tests/nextflow.config:

// Impose sensible resource limits for testing
process {
    withName: '.*' {
        cpus   = 2
        memory = 3.GB
        time   = 2.h
    }
}