runners: bench: Added litmus flag, default to disabled

The litmus benches are really only intended for introspection/debugging/
cool plots/etc. They're interesting to poke around with and cover a wide
range of littlefs's data-structures, but are not very rigorous.

To make this more clear for new users, added a new litmus flag for
benches:

  litmus = true

This doesn't change anything about how the bench is run, but serves as a
marker to hint that the bench is intended for non-rigorous benchmarking.

---

In the makefile, litmus tests are disabled by default at runtime
(--no-litmus). This is to limit `make bench` to benches that are useful
for performance comparisons.

With --no-litmus at runtime, the litmus benches are at least compiled
into the bench_runner, which should hopefully encourage keeping them up
to date with code changes. Eventually we should also run them in CI, but
only to check for runtime errors.

Unlike our tests, we're not really worried about compile time at the
moment due to how few/small our benches are.

benches/bench_btree.toml

@@ -23,6 +23,8 @@ defines.SEED = 42
 # 0x2 => lookup
 # 0x8 => usage
 defines.MASK = 0xb
+# not the most rigorous
+litmus = true
 in = 'lfs3.c'
 code = '''
     lfs3_t lfs3;
@@ -124,6 +126,8 @@ defines.SEED = 42
 # 0x4 => namelookup
 # 0x8 => usage
 defines.MASK = 0xf
+# not the most rigorous
+litmus = true
 in = 'lfs3.c'
 code = '''
     lfs3_t lfs3;

benches/bench_dir.toml

@@ -22,6 +22,8 @@ defines.SEED = 42
 # 0x4 => read
 # 0x8 => usage
 defines.MASK = 0xf
+# not the most rigorous
+litmus = true
 code = '''
     lfs3_t lfs3;
     lfs3_format(&lfs3, LFS3_F_RDWR, CFG) => 0;

benches/bench_file.toml

@@ -24,6 +24,8 @@ defines.SEED = 42
 # 0x2 => read
 # 0x4 => usage
 defines.MASK = 0x7
+# not the most rigorous
+litmus = true
 code = '''
     lfs3_t lfs3;
     lfs3_format(&lfs3, LFS3_F_RDWR, CFG) => 0;

benches/bench_rbyd.toml

@@ -27,6 +27,8 @@ defines.SEED = 42
 # 0x08 => lookup
 # 0x10 => usage
 defines.MASK = 0x1f
+# not the most rigorous
+litmus = true
 in = 'lfs3.c'
 code = '''
     lfs3_t lfs3;
@@ -138,6 +140,8 @@ defines.SEED = 42
 # 0x08 => lookup
 # 0x10 => usage
 defines.MASK = 0x1f
+# not the most rigorous
+litmus = true
 in = 'lfs3.c'
 code = '''
     lfs3_t lfs3;