Test harness fixes

[andrewbranch]

#54506 oversimplified things a little by always sending absolute paths to createProgram. When running tsc without a tsconfig file, root names for the program are actually CWD-relative, like src/index.ts. Compiler tests that use non-absolute // @Filename comments previously did this. With this PR,

• tests that have a tsconfig still send absolute root names to createProgram (as introduced in Process only tsconfig-referenced files in compiler tests with tsconfig files #54506)
• tests that don't have a tsconfig pass non-absolute root names to createProgram unless the // @Filename is specified absolute (reverting change from Process only tsconfig-referenced files in compiler tests with tsconfig files #54506)
• non-absolute // @Filename files have tests/cases/compiler prepended to them
• non-absolute paths in compiler options (e.g. // @outDir: out) have tests/cases/compiler prepended to them
• compilerOptions passed to createProgram then have paths converted to absolute,
  as always happens with tsc (Process only tsconfig-referenced files in compiler tests with tsconfig files #54506 missed this)

This causes some paths in baselines to show up with tests/cases/compiler added to them. It doesn’t seem like it’s necessary to keep that fake directory anymore. It would simplify things a lot to remove it. I’ll look into doing that in a subsequent commit or another PR.