feat: add support for fixtures

[not-matthias]

Normal output:

MyFixture/FooTest                                   0.000 ns        0.000 ns   1000000000000
MyFixture/BarTest                                   0.000 ns        0.000 ns   1000000000000
MyTemplatedFixture<int>/IntTest                     0.000 ns        0.000 ns   1000000000000
MyTemplatedFixture<double>/DoubleTest               0.000 ns        0.000 ns   1000000000000
MyTemplate1<int>/TestA                              0.000 ns        0.000 ns   1000000000000
MyTemplate2<int,double>/TestB                       0.000 ns        0.000 ns   1000000000000

Output with instrumentation enabled:

Checked: examples/google_benchmark/fixture_bench.hpp::example_namespace::MyFixture[FooTest]
Checked: examples/google_benchmark/fixture_bench.hpp::example_namespace::MyFixture[BarTest]
Checked: examples/google_benchmark/fixture_bench.hpp::example_namespace::MyTemplatedFixture[IntTest, int]
Checked: examples/google_benchmark/fixture_bench.hpp::example_namespace::MyTemplatedFixture[DoubleTest, double]
Checked: examples/google_benchmark/fixture_bench.hpp::example_namespace::MyTemplate1[TestA, int]
Checked: examples/google_benchmark/fixture_bench.hpp::example_namespace::MyTemplate2[TestB, int, double]

Minor detail: Should the format be MyFixture[TestA] or TestA[MyFixture]. Both make sense imo.

[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #7 will not alter performance

_{Comparing cod-668-support-fixtures-in-google-benchmark (772a2db) with main (1dda619)}

Summary

✅ 15 untouched benchmarks
🆕 6 new benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
🆕	BarTest[MyFixture]	N/A	6.2 µs	N/A
🆕	DoubleTest[MyTemplatedFixture, double]	N/A	6.3 µs	N/A
🆕	FooTest[MyFixture]	N/A	6.3 µs	N/A
🆕	IntTest[MyTemplatedFixture, int]	N/A	6.3 µs	N/A
🆕	TestA[MyTemplate1, int]	N/A	6.3 µs	N/A
🆕	TestB[MyTemplate2, int, double]	N/A	6.3 µs	N/A

[codspeed-hq]

CodSpeed Walltime Performance Report

Merging #7 will not alter performance

_{Comparing cod-668-support-fixtures-in-google-benchmark (772a2db) with main (1dda619)}

Summary

✅ 15 untouched benchmarks
🆕 6 new benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
🆕	BarTest[MyFixture]	N/A	3.6 ns	N/A
🆕	DoubleTest[MyTemplatedFixture, double]	N/A	3.8 ns	N/A
🆕	FooTest[MyFixture]	N/A	3.8 ns	N/A
🆕	IntTest[MyTemplatedFixture, int]	N/A	3.7 ns	N/A
🆕	TestA[MyTemplate1, int]	N/A	3.8 ns	N/A
🆕	TestB[MyTemplate2, int, double]	N/A	3.6 ns	N/A

[GuillaumeLagrange]

olgtm, let's go with test[fixture] as discussed in person

[GuillaumeLagrange]

lgtm